Now it copies in the current versions of BoxesRunTime and ScalaRunTime
and applies patches to them, and the whole build is automated.

  # This is the only thing I actually typed, the rest is fancy echo.
  $ test/instrumented/mkinstrumented.sh build

  % rm -rf /scratch/trunk1/test/instrumented/classes
  % cp /scratch/trunk1/test/instrumented/../../src/library/scala/runtime/BoxesRunTime.java /scratch/trunk1/test/instrumented/../../src/library/scala/runtime/ScalaRunTime.scala /scratch/trunk1/test/instrumented/library/scala/runtime
  % patch BoxesRunTime.java /scratch/trunk1/test/instrumented/boxes.patch
  patching file BoxesRunTime.java
  % patch ScalaRunTime.scala /scratch/trunk1/test/instrumented/srt.patch
  patching file ScalaRunTime.scala
  Hunk #3 succeeded at 63 (offset 23 lines).
  Hunk #4 succeeded at 78 (offset 23 lines).
  Hunk #5 succeeded at 81 (offset 23 lines).
  Hunk #6 succeeded at 96 (offset 23 lines).
  % /scratch/trunk1/test/instrumented/../../build/pack/bin/scalac -d /scratch/trunk1/test/instrumented/classes /scratch/trunk1/test/instrumented/library/scala/runtime/BoxesRunTime.java /scratch/trunk1/test/instrumented/library/scala/runtime/ScalaRunTime.scala
  % javac -cp /scratch/trunk1/test/instrumented/../../build/pack/lib/scala-library.jar -d /scratch/trunk1/test/instrumented/classes /scratch/trunk1/test/instrumented/library/scala/runtime/BoxesRunTime.java
  % cd /scratch/trunk1/test/instrumented/classes
  % jar cf instrumented.jar .
  % mv -f instrumented.jar /scratch/trunk1/test/instrumented/../../test/files/speclib
  /scratch/trunk1/test/files/speclib/instrumented.jar has been created.

Fixing the usecase suggested by Jason

Specific optimizations to findMember that have become possible because findMembers is its own function now.

Specific optimizations to findMember that have become possible because findMembers is its own function now.

Swathes of important logic are duplicated between `findMember`
and `findMembers` after they separated on grounds of irreconcilable
differences about how fast they should run:

    d905558 Variation #10 to optimze findMember
    fcb0c01 Attempt #9 to opimize findMember.
    71d2ceb Attempt #8 to opimize findMember.
    77e5692 Attempty #7 to optimize findMember
    275115e Fixing problem that caused fingerprints to fail in
    e94252e Attemmpt #6 to optimize findMember
    73e61b8 Attempt #5 to optimize findMember.
    04f0b65 Attempt #4 to optimize findMember
    0e3c70f Attempt #3 to optimize findMember
    41f4497 Attempt #2 to optimize findMember
    1a73aa0 Attempt #1 to optimize findMember

This didn't actually bear fruit, and the intervening years have
seen the implementations drift.

Now is the time to reunite them under the banner of `FindMemberBase`.

Each has a separate subclass to customise the behaviour. This is
primarily used by `findMember` to cache member types and to assemble
the resulting list of symbols in an low-allocation manner.

While there I have introduced some polymorphic calls, the call sites
are only bi-morphic, and our typical pattern of compilation involves
far more `findMember` calls, so I expect that JIT will keep the
virtual call cost to an absolute minimum.

Test results have been updated now that `findMembers` correctly
excludes constructors and doesn't inherit privates.

Coming up next: we can actually fix SI-7475!

Cut number of errors almost in half

Sip-23 unit tests

Manually tested with:

```
% cat sandbox/test.scala
package p {
  object X { def f(i: Int) = ??? ; def f(s: String) = ??? }
  object Main {
    val res = X.f(3.14)
  }
}

% qscalac  -Ytyper-debug sandbox/test.scala
|-- p EXPRmode-POLYmode-QUALmode (site: package <root>)
|    \-> p.type
|-- object X BYVALmode-EXPRmode (site: package p)
|    |-- super EXPRmode-POLYmode-QUALmode (silent: <init> in X)
|    |    |-- this EXPRmode (silent: <init> in X)
|    |    |    \-> p.X.type
|    |    \-> p.X.type
|    |-- def f BYVALmode-EXPRmode (site: object X)
|    |    |-- $qmark$qmark$qmark EXPRmode (site: method f in X)
|    |    |    \-> Nothing
|    |    |-- Int TYPEmode (site: value i in X)
|    |    |    \-> Int
|    |    |-- Int TYPEmode (site: value i in X)
|    |    |    \-> Int
|    |    \-> [def f] (i: Int)Nothing
|    |-- def f BYVALmode-EXPRmode (site: object X)
|    |    |-- $qmark$qmark$qmark EXPRmode (site: method f in X)
|    |    |    \-> Nothing
|    |    |-- String TYPEmode (site: value s in X)
|    |    |    [adapt] String is now a TypeTree(String)
|    |    |    \-> String
|    |    |-- String TYPEmode (site: value s in X)
|    |    |    [adapt] String is now a TypeTree(String)
|    |    |    \-> String
|    |    \-> [def f] (s: String)Nothing
|    \-> [object X] p.X.type
|-- object Main BYVALmode-EXPRmode (site: package p)
|    |-- X.f(3.14) EXPRmode (site: value res  in Main)
|    |    |-- X.f BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value res  in Main)
|    |    |    |-- X EXPRmode-POLYmode-QUALmode (silent: value res  in Main)
|    |    |    |    \-> p.X.type
|    |    |    \-> (s: String)Nothing <and> (i: Int)Nothing
|    |    |-- 3.14 BYVALmode-EXPRmode (silent: value res  in Main)
|    |    |    \-> Double(3.14)
|    |    [search #1] start `<?>`, searching for adaptation to pt=Double => String (silent: value res  in Main) implicits disabled
|    |    [search #2] start `<?>`, searching for adaptation to pt=(=> Double) => String (silent: value res  in Main) implicits disabled
|    |    [search #3] start `<?>`, searching for adaptation to pt=Double => Int (silent: value res  in Main) implicits disabled
|    |    1 implicits in companion scope
|    |    [search #4] start `<?>`, searching for adaptation to pt=(=> Double) => Int (silent: value res  in Main) implicits disabled
|    |    1 implicits in companion scope
|    |    second try: <error> and 3.14
|    |    [search #5] start `p.X.type`, searching for adaptation to pt=p.X.type => ?{def f(x$1: ? >: Double(3.14)): ?} (silent: value res  in Main) implicits disabled
|    |    [search #6] start `p.X.type`, searching for adaptation to pt=(=> p.X.type) => ?{def f(x$1: ? >: Double(3.14)): ?} (silent: value res  in Main) implicits disabled
sandbox/test.scala:4: error: overloaded method value f with alternatives:
  (s: String)Nothing <and>
  (i: Int)Nothing
 cannot be applied to (Double)
    val res = X.f(3.14)
                ^
```

avoid using macro-based StringContext.f to ease bootstrap

The following commit message is a squash of several commit messages.

- This is the 1st commit message:

Add position to stub error messages

Stub errors happen when we've started the initialization of a symbol but
key information of this symbol is missing (the information cannot be
found in any entry of the classpath not sources).

When this error happens, we better have a good error message with a
position to the place where the stub error came from. This commit goes
into this direction by adding a `pos` value to `StubSymbol` and filling
it in in all the use sites (especifically `UnPickler`).

This commit also changes some tests that test stub errors-related
issues. Concretely, `t6440` is using special Partest infrastructure and
doens't pretty print the position, while `t5148` which uses the
conventional infrastructure does. Hence the difference in the changes
for both tests.

- This is the commit message scala#2:

Add partest infrastructure to test stub errors

`StubErrorMessageTest` is the friend I introduce in this commit to help
state stub errors. The strategy to test them is easy and builds upon
previous concepts: we reuse `StoreReporterDirectTest` and add some
methods that will compile the code and simulate a missing classpath
entry by removing the class files from the class directory (the folder
where Scalac compiles to).

This first iteration allow us to programmatically check that stub errors
are emitted under certain conditions.

- This is the commit message scala#3:

Improve contents of stub error message

This commit does three things:

* Keep track of completing symbol while unpickling

  First, it removes the previous `symbolOnCompletion` definition to be
  more restrictive/clear and use only positions, since only positions are
  used to report the error (the rest of the information comes from the
  context of the `UnPickler`).

  Second, it adds a new variable called `lazyCompletingSymbol` that is
  responsible for keeping a reference to the symbol that produces the stub
  error. This symbol will usually (always?) come from the classpath
  entries and therefore we don't have its position (that's why we keep
  track of `symbolOnCompletion` as well). This is the one that we have to
  explicitly use in the stub error message, the culprit so to speak.

  Aside from these two changes, this commit modifies the existing tests
  that are affected by the change in the error message, which is more
  precise now, and adds new tests for stub errors that happen in complex
  inner cases and in return type of `MethodType`.

* Check that order of initialization is correct

  With the changes introduced previously to keep track of position of
  symbols coming from source files, we may ask ourselves: is this going to
  work always? What happens if two symbols the initialization of two
  symbols is intermingled and the stub error message gets the wrong
  position?

  This commit adds a test case and modifications to the test
  infrastructure to double check empirically that this does not happen.
  Usually, this interaction in symbol initialization won't happen because
  the `UnPickler` will lazily load all the buckets necessary for a symbol
  to be truly initialized, with the pertinent addresses from which this
  information has to be deserialized. This ensures that this operation is
  atomic and no other symbol initialization can happen in the meantime.

  Even though the previous paragraph is the feeling I got from reading the
  sources, this commit creates a test to double-check it. My attempt to be
  better safe than sorry.

* Improve contents of the stub error message

  This commit modifies the format of the previous stub error message by
  being more precise in its formulation. It follows the structured format:

  ```
  s"""|Symbol '${name.nameKind} ${owner.fullName}.$name' is missing from the classpath.
      |This symbol is required by '${lazyCompletingSymbol.kindString} ${lazyCompletingSymbol.fullName}'.
  ```

  This format has the advantage that is more readable and explicit on
  what's happening. First, we report what is missing. Then, why it was
  required. Hopefully, people working on direct dependencies will find the
  new message friendlier.

Having a good test suite to check the previously added code is
important. This commit checks that stub errors happen in presence of
well-known and widely used Scala features. These include:

* Higher kinded types.
* Type definitions.
* Inheritance and subclasses.
* Typeclasses and implicits.

- This is the commit message scala#4:

Use `lastTreeToTyper` to get better positions

The previous strategy to get the last user-defined position for knowing
what was the root cause (the trigger) of stub errors relied on
instrumenting `def info`.

This instrumentation, while easy to implement, is inefficient since we
register the positions for symbols that are already completed.

However, we cannot do it only for uncompleted symbols (!hasCompleteInfo)
because the positions won't be correct anymore -- definitions using stub
symbols (val b = new B) are for the compiler completed, but their use
throws stub errors. This means that if we initialize symbols between a
definition and its use, we'll use their positions instead of the
position of `b`.

To work around this we use `lastTreeToTyper`. We assume that stub errors
will be thrown by Typer at soonest.

The benefit of this approach is better error messages. The positions
used in them are now as concrete as possible since they point to the
exact tree that **uses** a symbol, instead of the one that **defines**
it. Have a look at `StubErrorComplexInnerClass` for an example.

This commit removes the previous infrastructure and replaces it by the
new one. It also removes the fields positions from the subclasses of
`StubSymbol`s.

- This is the commit message scala#5:

Keep track of completing symbols

Make sure that cycles don't happen by keeping track of all the
symbols that are being completed by `completeInternal`. Stub errors only
need the last completing symbols, but the whole stack of symbols may
be useful to reporting other error like cyclic initialization issues.

I've added this per Jason's suggestion. I've implemented with a list
because `remove` in an array buffer is linear. Array was not an option
because I would need to resize it myself. I think that even though list
is not as efficient memory-wise, it probably doesn't matter since the
stack will usually be small.

- This is the commit message scala#6:

Remove `isPackage` from `newStubSymbol`

Remove `isPackage` since in 2.12.x its value is not used.

The following commit message is a squash of several commit messages.

- This is the 1st commit message:

Add position to stub error messages

Stub errors happen when we've started the initialization of a symbol but
key information of this symbol is missing (the information cannot be
found in any entry of the classpath not sources).

When this error happens, we better have a good error message with a
position to the place where the stub error came from. This commit goes
into this direction by adding a `pos` value to `StubSymbol` and filling
it in in all the use sites (especifically `UnPickler`).

This commit also changes some tests that test stub errors-related
issues. Concretely, `t6440` is using special Partest infrastructure and
doens't pretty print the position, while `t5148` which uses the
conventional infrastructure does. Hence the difference in the changes
for both tests.

- This is the commit message scala#2:

Add partest infrastructure to test stub errors

`StubErrorMessageTest` is the friend I introduce in this commit to help
state stub errors. The strategy to test them is easy and builds upon
previous concepts: we reuse `StoreReporterDirectTest` and add some
methods that will compile the code and simulate a missing classpath
entry by removing the class files from the class directory (the folder
where Scalac compiles to).

This first iteration allow us to programmatically check that stub errors
are emitted under certain conditions.

- This is the commit message scala#3:

Improve contents of stub error message

This commit does three things:

* Keep track of completing symbol while unpickling

  First, it removes the previous `symbolOnCompletion` definition to be
  more restrictive/clear and use only positions, since only positions are
  used to report the error (the rest of the information comes from the
  context of the `UnPickler`).

  Second, it adds a new variable called `lazyCompletingSymbol` that is
  responsible for keeping a reference to the symbol that produces the stub
  error. This symbol will usually (always?) come from the classpath
  entries and therefore we don't have its position (that's why we keep
  track of `symbolOnCompletion` as well). This is the one that we have to
  explicitly use in the stub error message, the culprit so to speak.

  Aside from these two changes, this commit modifies the existing tests
  that are affected by the change in the error message, which is more
  precise now, and adds new tests for stub errors that happen in complex
  inner cases and in return type of `MethodType`.

* Check that order of initialization is correct

  With the changes introduced previously to keep track of position of
  symbols coming from source files, we may ask ourselves: is this going to
  work always? What happens if two symbols the initialization of two
  symbols is intermingled and the stub error message gets the wrong
  position?

  This commit adds a test case and modifications to the test
  infrastructure to double check empirically that this does not happen.
  Usually, this interaction in symbol initialization won't happen because
  the `UnPickler` will lazily load all the buckets necessary for a symbol
  to be truly initialized, with the pertinent addresses from which this
  information has to be deserialized. This ensures that this operation is
  atomic and no other symbol initialization can happen in the meantime.

  Even though the previous paragraph is the feeling I got from reading the
  sources, this commit creates a test to double-check it. My attempt to be
  better safe than sorry.

* Improve contents of the stub error message

  This commit modifies the format of the previous stub error message by
  being more precise in its formulation. It follows the structured format:

  ```
  s"""|Symbol '${name.nameKind} ${owner.fullName}.$name' is missing from the classpath.
      |This symbol is required by '${lazyCompletingSymbol.kindString} ${lazyCompletingSymbol.fullName}'.
  ```

  This format has the advantage that is more readable and explicit on
  what's happening. First, we report what is missing. Then, why it was
  required. Hopefully, people working on direct dependencies will find the
  new message friendlier.

Having a good test suite to check the previously added code is
important. This commit checks that stub errors happen in presence of
well-known and widely used Scala features. These include:

* Higher kinded types.
* Type definitions.
* Inheritance and subclasses.
* Typeclasses and implicits.

- This is the commit message scala#4:

Use `lastTreeToTyper` to get better positions

The previous strategy to get the last user-defined position for knowing
what was the root cause (the trigger) of stub errors relied on
instrumenting `def info`.

This instrumentation, while easy to implement, is inefficient since we
register the positions for symbols that are already completed.

However, we cannot do it only for uncompleted symbols (!hasCompleteInfo)
because the positions won't be correct anymore -- definitions using stub
symbols (val b = new B) are for the compiler completed, but their use
throws stub errors. This means that if we initialize symbols between a
definition and its use, we'll use their positions instead of the
position of `b`.

To work around this we use `lastTreeToTyper`. We assume that stub errors
will be thrown by Typer at soonest.

The benefit of this approach is better error messages. The positions
used in them are now as concrete as possible since they point to the
exact tree that **uses** a symbol, instead of the one that **defines**
it. Have a look at `StubErrorComplexInnerClass` for an example.

This commit removes the previous infrastructure and replaces it by the
new one. It also removes the fields positions from the subclasses of
`StubSymbol`s.

- This is the commit message scala#5:

Keep track of completing symbols

Make sure that cycles don't happen by keeping track of all the
symbols that are being completed by `completeInternal`. Stub errors only
need the last completing symbols, but the whole stack of symbols may
be useful to reporting other error like cyclic initialization issues.

I've added this per Jason's suggestion. I've implemented with a list
because `remove` in an array buffer is linear. Array was not an option
because I would need to resize it myself. I think that even though list
is not as efficient memory-wise, it probably doesn't matter since the
stack will usually be small.

- This is the commit message scala#6:

Remove `isPackage` from `newStubSymbol`

Remove `isPackage` since in 2.12.x its value is not used.

The following commit message is a squash of several commit messages.

- This is the 1st commit message:

Add position to stub error messages

Stub errors happen when we've started the initialization of a symbol but
key information of this symbol is missing (the information cannot be
found in any entry of the classpath not sources).

When this error happens, we better have a good error message with a
position to the place where the stub error came from. This commit goes
into this direction by adding a `pos` value to `StubSymbol` and filling
it in in all the use sites (especifically `UnPickler`).

This commit also changes some tests that test stub errors-related
issues. Concretely, `t6440` is using special Partest infrastructure and
doens't pretty print the position, while `t5148` which uses the
conventional infrastructure does. Hence the difference in the changes
for both tests.

- This is the commit message scala#2:

Add partest infrastructure to test stub errors

`StubErrorMessageTest` is the friend I introduce in this commit to help
state stub errors. The strategy to test them is easy and builds upon
previous concepts: we reuse `StoreReporterDirectTest` and add some
methods that will compile the code and simulate a missing classpath
entry by removing the class files from the class directory (the folder
where Scalac compiles to).

This first iteration allow us to programmatically check that stub errors
are emitted under certain conditions.

- This is the commit message scala#3:

Improve contents of stub error message

This commit does three things:

* Keep track of completing symbol while unpickling

  First, it removes the previous `symbolOnCompletion` definition to be
  more restrictive/clear and use only positions, since only positions are
  used to report the error (the rest of the information comes from the
  context of the `UnPickler`).

  Second, it adds a new variable called `lazyCompletingSymbol` that is
  responsible for keeping a reference to the symbol that produces the stub
  error. This symbol will usually (always?) come from the classpath
  entries and therefore we don't have its position (that's why we keep
  track of `symbolOnCompletion` as well). This is the one that we have to
  explicitly use in the stub error message, the culprit so to speak.

  Aside from these two changes, this commit modifies the existing tests
  that are affected by the change in the error message, which is more
  precise now, and adds new tests for stub errors that happen in complex
  inner cases and in return type of `MethodType`.

* Check that order of initialization is correct

  With the changes introduced previously to keep track of position of
  symbols coming from source files, we may ask ourselves: is this going to
  work always? What happens if two symbols the initialization of two
  symbols is intermingled and the stub error message gets the wrong
  position?

  This commit adds a test case and modifications to the test
  infrastructure to double check empirically that this does not happen.
  Usually, this interaction in symbol initialization won't happen because
  the `UnPickler` will lazily load all the buckets necessary for a symbol
  to be truly initialized, with the pertinent addresses from which this
  information has to be deserialized. This ensures that this operation is
  atomic and no other symbol initialization can happen in the meantime.

  Even though the previous paragraph is the feeling I got from reading the
  sources, this commit creates a test to double-check it. My attempt to be
  better safe than sorry.

* Improve contents of the stub error message

  This commit modifies the format of the previous stub error message by
  being more precise in its formulation. It follows the structured format:

  ```
  s"""|Symbol '${name.nameKind} ${owner.fullName}.$name' is missing from the classpath.
      |This symbol is required by '${lazyCompletingSymbol.kindString} ${lazyCompletingSymbol.fullName}'.
  ```

  This format has the advantage that is more readable and explicit on
  what's happening. First, we report what is missing. Then, why it was
  required. Hopefully, people working on direct dependencies will find the
  new message friendlier.

Having a good test suite to check the previously added code is
important. This commit checks that stub errors happen in presence of
well-known and widely used Scala features. These include:

* Higher kinded types.
* Type definitions.
* Inheritance and subclasses.
* Typeclasses and implicits.

- This is the commit message scala#4:

Use `lastTreeToTyper` to get better positions

The previous strategy to get the last user-defined position for knowing
what was the root cause (the trigger) of stub errors relied on
instrumenting `def info`.

This instrumentation, while easy to implement, is inefficient since we
register the positions for symbols that are already completed.

However, we cannot do it only for uncompleted symbols (!hasCompleteInfo)
because the positions won't be correct anymore -- definitions using stub
symbols (val b = new B) are for the compiler completed, but their use
throws stub errors. This means that if we initialize symbols between a
definition and its use, we'll use their positions instead of the
position of `b`.

To work around this we use `lastTreeToTyper`. We assume that stub errors
will be thrown by Typer at soonest.

The benefit of this approach is better error messages. The positions
used in them are now as concrete as possible since they point to the
exact tree that **uses** a symbol, instead of the one that **defines**
it. Have a look at `StubErrorComplexInnerClass` for an example.

This commit removes the previous infrastructure and replaces it by the
new one. It also removes the fields positions from the subclasses of
`StubSymbol`s.

- This is the commit message scala#5:

Keep track of completing symbols

Make sure that cycles don't happen by keeping track of all the
symbols that are being completed by `completeInternal`. Stub errors only
need the last completing symbols, but the whole stack of symbols may
be useful to reporting other error like cyclic initialization issues.

I've added this per Jason's suggestion. I've implemented with a list
because `remove` in an array buffer is linear. Array was not an option
because I would need to resize it myself. I think that even though list
is not as efficient memory-wise, it probably doesn't matter since the
stack will usually be small.

- This is the commit message scala#6:

Remove `isPackage` from `newStubSymbol`

Remove `isPackage` since in 2.12.x its value is not used.

The following commit message is a squash of several commit messages.

- This is the 1st commit message:

Add position to stub error messages

Stub errors happen when we've started the initialization of a symbol but
key information of this symbol is missing (the information cannot be
found in any entry of the classpath not sources).

When this error happens, we better have a good error message with a
position to the place where the stub error came from. This commit goes
into this direction by adding a `pos` value to `StubSymbol` and filling
it in in all the use sites (especifically `UnPickler`).

This commit also changes some tests that test stub errors-related
issues. Concretely, `t6440` is using special Partest infrastructure and
doens't pretty print the position, while `t5148` which uses the
conventional infrastructure does. Hence the difference in the changes
for both tests.

- This is the commit message scala#2:

Add partest infrastructure to test stub errors

`StubErrorMessageTest` is the friend I introduce in this commit to help
state stub errors. The strategy to test them is easy and builds upon
previous concepts: we reuse `StoreReporterDirectTest` and add some
methods that will compile the code and simulate a missing classpath
entry by removing the class files from the class directory (the folder
where Scalac compiles to).

This first iteration allow us to programmatically check that stub errors
are emitted under certain conditions.

- This is the commit message scala#3:

Improve contents of stub error message

This commit does three things:

* Keep track of completing symbol while unpickling

  First, it removes the previous `symbolOnCompletion` definition to be
  more restrictive/clear and use only positions, since only positions are
  used to report the error (the rest of the information comes from the
  context of the `UnPickler`).

  Second, it adds a new variable called `lazyCompletingSymbol` that is
  responsible for keeping a reference to the symbol that produces the stub
  error. This symbol will usually (always?) come from the classpath
  entries and therefore we don't have its position (that's why we keep
  track of `symbolOnCompletion` as well). This is the one that we have to
  explicitly use in the stub error message, the culprit so to speak.

  Aside from these two changes, this commit modifies the existing tests
  that are affected by the change in the error message, which is more
  precise now, and adds new tests for stub errors that happen in complex
  inner cases and in return type of `MethodType`.

* Check that order of initialization is correct

  With the changes introduced previously to keep track of position of
  symbols coming from source files, we may ask ourselves: is this going to
  work always? What happens if two symbols the initialization of two
  symbols is intermingled and the stub error message gets the wrong
  position?

  This commit adds a test case and modifications to the test
  infrastructure to double check empirically that this does not happen.
  Usually, this interaction in symbol initialization won't happen because
  the `UnPickler` will lazily load all the buckets necessary for a symbol
  to be truly initialized, with the pertinent addresses from which this
  information has to be deserialized. This ensures that this operation is
  atomic and no other symbol initialization can happen in the meantime.

  Even though the previous paragraph is the feeling I got from reading the
  sources, this commit creates a test to double-check it. My attempt to be
  better safe than sorry.

* Improve contents of the stub error message

  This commit modifies the format of the previous stub error message by
  being more precise in its formulation. It follows the structured format:

  ```
  s"""|Symbol '${name.nameKind} ${owner.fullName}.$name' is missing from the classpath.
      |This symbol is required by '${lazyCompletingSymbol.kindString} ${lazyCompletingSymbol.fullName}'.
  ```

  This format has the advantage that is more readable and explicit on
  what's happening. First, we report what is missing. Then, why it was
  required. Hopefully, people working on direct dependencies will find the
  new message friendlier.

Having a good test suite to check the previously added code is
important. This commit checks that stub errors happen in presence of
well-known and widely used Scala features. These include:

* Higher kinded types.
* Type definitions.
* Inheritance and subclasses.
* Typeclasses and implicits.

- This is the commit message scala#4:

Use `lastTreeToTyper` to get better positions

The previous strategy to get the last user-defined position for knowing
what was the root cause (the trigger) of stub errors relied on
instrumenting `def info`.

This instrumentation, while easy to implement, is inefficient since we
register the positions for symbols that are already completed.

However, we cannot do it only for uncompleted symbols (!hasCompleteInfo)
because the positions won't be correct anymore -- definitions using stub
symbols (val b = new B) are for the compiler completed, but their use
throws stub errors. This means that if we initialize symbols between a
definition and its use, we'll use their positions instead of the
position of `b`.

To work around this we use `lastTreeToTyper`. We assume that stub errors
will be thrown by Typer at soonest.

The benefit of this approach is better error messages. The positions
used in them are now as concrete as possible since they point to the
exact tree that **uses** a symbol, instead of the one that **defines**
it. Have a look at `StubErrorComplexInnerClass` for an example.

This commit removes the previous infrastructure and replaces it by the
new one. It also removes the fields positions from the subclasses of
`StubSymbol`s.

- This is the commit message scala#5:

Keep track of completing symbols

Make sure that cycles don't happen by keeping track of all the
symbols that are being completed by `completeInternal`. Stub errors only
need the last completing symbols, but the whole stack of symbols may
be useful to reporting other error like cyclic initialization issues.

I've added this per Jason's suggestion. I've implemented with a list
because `remove` in an array buffer is linear. Array was not an option
because I would need to resize it myself. I think that even though list
is not as efficient memory-wise, it probably doesn't matter since the
stack will usually be small.

- This is the commit message scala#6:

Remove `isPackage` from `newStubSymbol`

Remove `isPackage` since in 2.12.x its value is not used.

The following commit message is a squash of several commit messages.

- This is the 1st commit message:

Add position to stub error messages

Stub errors happen when we've started the initialization of a symbol but
key information of this symbol is missing (the information cannot be
found in any entry of the classpath not sources).

When this error happens, we better have a good error message with a
position to the place where the stub error came from. This commit goes
into this direction by adding a `pos` value to `StubSymbol` and filling
it in in all the use sites (especifically `UnPickler`).

This commit also changes some tests that test stub errors-related
issues. Concretely, `t6440` is using special Partest infrastructure and
doens't pretty print the position, while `t5148` which uses the
conventional infrastructure does. Hence the difference in the changes
for both tests.

- This is the commit message scala#2:

Add partest infrastructure to test stub errors

`StubErrorMessageTest` is the friend I introduce in this commit to help
state stub errors. The strategy to test them is easy and builds upon
previous concepts: we reuse `StoreReporterDirectTest` and add some
methods that will compile the code and simulate a missing classpath
entry by removing the class files from the class directory (the folder
where Scalac compiles to).

This first iteration allow us to programmatically check that stub errors
are emitted under certain conditions.

- This is the commit message scala#3:

Improve contents of stub error message

This commit does three things:

* Keep track of completing symbol while unpickling

  First, it removes the previous `symbolOnCompletion` definition to be
  more restrictive/clear and use only positions, since only positions are
  used to report the error (the rest of the information comes from the
  context of the `UnPickler`).

  Second, it adds a new variable called `lazyCompletingSymbol` that is
  responsible for keeping a reference to the symbol that produces the stub
  error. This symbol will usually (always?) come from the classpath
  entries and therefore we don't have its position (that's why we keep
  track of `symbolOnCompletion` as well). This is the one that we have to
  explicitly use in the stub error message, the culprit so to speak.

  Aside from these two changes, this commit modifies the existing tests
  that are affected by the change in the error message, which is more
  precise now, and adds new tests for stub errors that happen in complex
  inner cases and in return type of `MethodType`.

* Check that order of initialization is correct

  With the changes introduced previously to keep track of position of
  symbols coming from source files, we may ask ourselves: is this going to
  work always? What happens if two symbols the initialization of two
  symbols is intermingled and the stub error message gets the wrong
  position?

  This commit adds a test case and modifications to the test
  infrastructure to double check empirically that this does not happen.
  Usually, this interaction in symbol initialization won't happen because
  the `UnPickler` will lazily load all the buckets necessary for a symbol
  to be truly initialized, with the pertinent addresses from which this
  information has to be deserialized. This ensures that this operation is
  atomic and no other symbol initialization can happen in the meantime.

  Even though the previous paragraph is the feeling I got from reading the
  sources, this commit creates a test to double-check it. My attempt to be
  better safe than sorry.

* Improve contents of the stub error message

  This commit modifies the format of the previous stub error message by
  being more precise in its formulation. It follows the structured format:

  ```
  s"""|Symbol '${name.nameKind} ${owner.fullName}.$name' is missing from the classpath.
      |This symbol is required by '${lazyCompletingSymbol.kindString} ${lazyCompletingSymbol.fullName}'.
  ```

  This format has the advantage that is more readable and explicit on
  what's happening. First, we report what is missing. Then, why it was
  required. Hopefully, people working on direct dependencies will find the
  new message friendlier.

Having a good test suite to check the previously added code is
important. This commit checks that stub errors happen in presence of
well-known and widely used Scala features. These include:

* Higher kinded types.
* Type definitions.
* Inheritance and subclasses.
* Typeclasses and implicits.

- This is the commit message scala#4:

Use `lastTreeToTyper` to get better positions

The previous strategy to get the last user-defined position for knowing
what was the root cause (the trigger) of stub errors relied on
instrumenting `def info`.

This instrumentation, while easy to implement, is inefficient since we
register the positions for symbols that are already completed.

However, we cannot do it only for uncompleted symbols (!hasCompleteInfo)
because the positions won't be correct anymore -- definitions using stub
symbols (val b = new B) are for the compiler completed, but their use
throws stub errors. This means that if we initialize symbols between a
definition and its use, we'll use their positions instead of the
position of `b`.

To work around this we use `lastTreeToTyper`. We assume that stub errors
will be thrown by Typer at soonest.

The benefit of this approach is better error messages. The positions
used in them are now as concrete as possible since they point to the
exact tree that **uses** a symbol, instead of the one that **defines**
it. Have a look at `StubErrorComplexInnerClass` for an example.

This commit removes the previous infrastructure and replaces it by the
new one. It also removes the fields positions from the subclasses of
`StubSymbol`s.

- This is the commit message scala#5:

Keep track of completing symbols

Make sure that cycles don't happen by keeping track of all the
symbols that are being completed by `completeInternal`. Stub errors only
need the last completing symbols, but the whole stack of symbols may
be useful to reporting other error like cyclic initialization issues.

I've added this per Jason's suggestion. I've implemented with a list
because `remove` in an array buffer is linear. Array was not an option
because I would need to resize it myself. I think that even though list
is not as efficient memory-wise, it probably doesn't matter since the
stack will usually be small.

- This is the commit message scala#6:

Remove `isPackage` from `newStubSymbol`

Remove `isPackage` since in 2.12.x its value is not used.

Make it clearer that the `iterator` method will return a fresh Iterator at each call

Rewritten from sbt/zinc@bba1b3f

Reify Tasty Names, which allows to reduce coupling

  - Avoid lambda allocation in getOrElseUpdate call
  - Avoid intermediate Tuple2 instances when iterating through
    the mutable map.
  - Avoid allocating Map1-4 during building the result if
    the size of the grouped exceeds 4. Intead build a HashMap
    directly.

```
./benchdb list --limit 2 && ./benchdb results --run 5 --run 6 --pivot size  && ./benchdb results --run 5 --run 6 --pivot size --metric "·gc.alloc.rate.norm"
┏━━━━┳━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━┓
┃ ID ┃ Timestamp           ┃ Msg              ┃
┣━━━━╋━━━━━━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━━━┫
┃  6 ┃ 2020-08-24 07:14:36 ┃ PR 8948          ┃
┃  5 ┃ 2020-08-24 07:06:47 ┃ PR 8948 Baseline ┃
┗━━━━┻━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━━━┛
2 test runs found (limit reached).
┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━┳━━━━━┳━━━━━━━━━━┳━━━━━━━━━┳━━━━━━━━━━━┳━━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━┓
┃                                                (size) ┃                ┃                ┃      ┃     ┃        128         ┃         512         ┃         2048          ┃         8192          ┃       ┃
┃ Benchmark                                             ┃ (hashCodeCost) ┃ (maxNumGroups) ┃ Mode ┃ Cnt ┃    Score ┃   Error ┃     Score ┃   Error ┃      Score ┃    Error ┃      Score ┃    Error ┃ Units ┃
┣━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━╋━━━━━╋━━━━━━━━━━╋━━━━━━━━━╋━━━━━━━━━━━╋━━━━━━━━━╋━━━━━━━━━━━━╋━━━━━━━━━━╋━━━━━━━━━━━━╋━━━━━━━━━━╋━━━━━━━┫
┃ scala#5:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 8631.963 ┃ 116.790 ┃ 30969.939 ┃ 456.711 ┃ 119820.013 ┃ 1741.671 ┃ 480813.306 ┃ 7268.723 ┃ ns/op ┃
┃ scala#6:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 8287.717 ┃ 135.038 ┃ 30672.551 ┃ 540.409 ┃ 119356.530 ┃ 1592.624 ┃ 477570.346 ┃ 6365.010 ┃ ns/op ┃
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━┻━━━━━┻━━━━━━━━━━┻━━━━━━━━━┻━━━━━━━━━━━┻━━━━━━━━━┻━━━━━━━━━━━━┻━━━━━━━━━━┻━━━━━━━━━━━━┻━━━━━━━━━━┻━━━━━━━┛
┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━┳━━━━━┳━━━━━━━━━━┳━━━━━━━┳━━━━━━━━━━━┳━━━━━━━┳━━━━━━━━━━━┳━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━┳━━━━━━━┓
┃                                                (size) ┃                ┃                ┃      ┃     ┃       128        ┃        512        ┃        2048        ┃        8192        ┃       ┃
┃ Benchmark [·gc.alloc.rate.norm]                       ┃ (hashCodeCost) ┃ (maxNumGroups) ┃ Mode ┃ Cnt ┃    Score ┃ Error ┃     Score ┃ Error ┃     Score ┃  Error ┃      Score ┃ Error ┃ Units ┃
┣━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━╋━━━━━╋━━━━━━━━━━╋━━━━━━━╋━━━━━━━━━━━╋━━━━━━━╋━━━━━━━━━━━╋━━━━━━━━╋━━━━━━━━━━━━╋━━━━━━━╋━━━━━━━┫
┃ scala#5:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 2192.014 ┃ 0.022 ┃ 13712.051 ┃ 0.076 ┃ 50949.533 ┃ 56.381 ┃ 198544.803 ┃ 1.213 ┃ B/op  ┃
┃ scala#6:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 1896.014 ┃ 0.021 ┃  5224.051 ┃ 0.077 ┃ 17768.204 ┃  0.305 ┃  67176.786 ┃ 1.180 ┃ B/op  ┃
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━┻━━━━━┻━━━━━━━━━━┻━━━━━━━┻━━━━━━━━━━━┻━━━━━━━┻━━━━━━━━━━━┻━━━━━━━━┻━━━━━━━━━━━━┻━━━━━━━┻━━━━━━━┛
```

  - Avoid lambda allocation in getOrElseUpdate call
  - Avoid intermediate Tuple2 instances when iterating through
    the mutable map.
  - Avoid allocating Map1-4 during building the result if
    the size of the grouped exceeds 4. Intead build a HashMap
    directly.

```
./benchdb list --limit 2 && ./benchdb results --run 5 --run 6 --pivot size  && ./benchdb results --run 5 --run 6 --pivot size --metric "·gc.alloc.rate.norm"
┏━━━━┳━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━┓
┃ ID ┃ Timestamp           ┃ Msg              ┃
┣━━━━╋━━━━━━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━━━┫
┃  6 ┃ 2020-08-24 07:14:36 ┃ PR 8948          ┃
┃  5 ┃ 2020-08-24 07:06:47 ┃ PR 8948 Baseline ┃
┗━━━━┻━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━━━┛
2 test runs found (limit reached).
┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━┳━━━━━┳━━━━━━━━━━┳━━━━━━━━━┳━━━━━━━━━━━┳━━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━┓
┃                                                (size) ┃                ┃                ┃      ┃     ┃        128         ┃         512         ┃         2048          ┃         8192          ┃       ┃
┃ Benchmark                                             ┃ (hashCodeCost) ┃ (maxNumGroups) ┃ Mode ┃ Cnt ┃    Score ┃   Error ┃     Score ┃   Error ┃      Score ┃    Error ┃      Score ┃    Error ┃ Units ┃
┣━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━╋━━━━━╋━━━━━━━━━━╋━━━━━━━━━╋━━━━━━━━━━━╋━━━━━━━━━╋━━━━━━━━━━━━╋━━━━━━━━━━╋━━━━━━━━━━━━╋━━━━━━━━━━╋━━━━━━━┫
┃ #5:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 8631.963 ┃ 116.790 ┃ 30969.939 ┃ 456.711 ┃ 119820.013 ┃ 1741.671 ┃ 480813.306 ┃ 7268.723 ┃ ns/op ┃
┃ #6:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 8287.717 ┃ 135.038 ┃ 30672.551 ┃ 540.409 ┃ 119356.530 ┃ 1592.624 ┃ 477570.346 ┃ 6365.010 ┃ ns/op ┃
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━┻━━━━━┻━━━━━━━━━━┻━━━━━━━━━┻━━━━━━━━━━━┻━━━━━━━━━┻━━━━━━━━━━━━┻━━━━━━━━━━┻━━━━━━━━━━━━┻━━━━━━━━━━┻━━━━━━━┛
┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━┳━━━━━┳━━━━━━━━━━┳━━━━━━━┳━━━━━━━━━━━┳━━━━━━━┳━━━━━━━━━━━┳━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━┳━━━━━━━┓
┃                                                (size) ┃                ┃                ┃      ┃     ┃       128        ┃        512        ┃        2048        ┃        8192        ┃       ┃
┃ Benchmark [·gc.alloc.rate.norm]                       ┃ (hashCodeCost) ┃ (maxNumGroups) ┃ Mode ┃ Cnt ┃    Score ┃ Error ┃     Score ┃ Error ┃     Score ┃  Error ┃      Score ┃ Error ┃ Units ┃
┣━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━╋━━━━━╋━━━━━━━━━━╋━━━━━━━╋━━━━━━━━━━━╋━━━━━━━╋━━━━━━━━━━━╋━━━━━━━━╋━━━━━━━━━━━━╋━━━━━━━╋━━━━━━━┫
┃ #5:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 2192.014 ┃ 0.022 ┃ 13712.051 ┃ 0.076 ┃ 50949.533 ┃ 56.381 ┃ 198544.803 ┃ 1.213 ┃ B/op  ┃
┃ #6:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 1896.014 ┃ 0.021 ┃  5224.051 ┃ 0.077 ┃ 17768.204 ┃  0.305 ┃  67176.786 ┃ 1.180 ┃ B/op  ┃
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━┻━━━━━┻━━━━━━━━━━┻━━━━━━━┻━━━━━━━━━━━┻━━━━━━━┻━━━━━━━━━━━┻━━━━━━━━┻━━━━━━━━━━━━┻━━━━━━━┻━━━━━━━┛
```

  - Avoid lambda allocation in getOrElseUpdate call
  - Avoid intermediate Tuple2 instances when iterating through
    the mutable map.
  - Avoid allocating Map1-4 during building the result if
    the size of the grouped exceeds 4. Intead build a HashMap
    directly.

```
./benchdb list --limit 2 && ./benchdb results --run 5 --run 6 --pivot size  && ./benchdb results --run 5 --run 6 --pivot size --metric "·gc.alloc.rate.norm"
┏━━━━┳━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━┓
┃ ID ┃ Timestamp           ┃ Msg              ┃
┣━━━━╋━━━━━━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━━━┫
┃  6 ┃ 2020-08-24 07:14:36 ┃ PR 8948          ┃
┃  5 ┃ 2020-08-24 07:06:47 ┃ PR 8948 Baseline ┃
┗━━━━┻━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━━━┛
2 test runs found (limit reached).
┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━┳━━━━━┳━━━━━━━━━━┳━━━━━━━━━┳━━━━━━━━━━━┳━━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━┓
┃                                                (size) ┃                ┃                ┃      ┃     ┃        128         ┃         512         ┃         2048          ┃         8192          ┃       ┃
┃ Benchmark                                             ┃ (hashCodeCost) ┃ (maxNumGroups) ┃ Mode ┃ Cnt ┃    Score ┃   Error ┃     Score ┃   Error ┃      Score ┃    Error ┃      Score ┃    Error ┃ Units ┃
┣━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━╋━━━━━╋━━━━━━━━━━╋━━━━━━━━━╋━━━━━━━━━━━╋━━━━━━━━━╋━━━━━━━━━━━━╋━━━━━━━━━━╋━━━━━━━━━━━━╋━━━━━━━━━━╋━━━━━━━┫
┃ scala/scala#5:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 8631.963 ┃ 116.790 ┃ 30969.939 ┃ 456.711 ┃ 119820.013 ┃ 1741.671 ┃ 480813.306 ┃ 7268.723 ┃ ns/op ┃
┃ scala/scala#6:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 8287.717 ┃ 135.038 ┃ 30672.551 ┃ 540.409 ┃ 119356.530 ┃ 1592.624 ┃ 477570.346 ┃ 6365.010 ┃ ns/op ┃
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━┻━━━━━┻━━━━━━━━━━┻━━━━━━━━━┻━━━━━━━━━━━┻━━━━━━━━━┻━━━━━━━━━━━━┻━━━━━━━━━━┻━━━━━━━━━━━━┻━━━━━━━━━━┻━━━━━━━┛
┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━┳━━━━━┳━━━━━━━━━━┳━━━━━━━┳━━━━━━━━━━━┳━━━━━━━┳━━━━━━━━━━━┳━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━┳━━━━━━━┓
┃                                                (size) ┃                ┃                ┃      ┃     ┃       128        ┃        512        ┃        2048        ┃        8192        ┃       ┃
┃ Benchmark [·gc.alloc.rate.norm]                       ┃ (hashCodeCost) ┃ (maxNumGroups) ┃ Mode ┃ Cnt ┃    Score ┃ Error ┃     Score ┃ Error ┃     Score ┃  Error ┃      Score ┃ Error ┃ Units ┃
┣━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━╋━━━━━╋━━━━━━━━━━╋━━━━━━━╋━━━━━━━━━━━╋━━━━━━━╋━━━━━━━━━━━╋━━━━━━━━╋━━━━━━━━━━━━╋━━━━━━━╋━━━━━━━┫
┃ scala/scala#5:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 2192.014 ┃ 0.022 ┃ 13712.051 ┃ 0.076 ┃ 50949.533 ┃ 56.381 ┃ 198544.803 ┃ 1.213 ┃ B/op  ┃
┃ scala/scala#6:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 1896.014 ┃ 0.021 ┃  5224.051 ┃ 0.077 ┃ 17768.204 ┃  0.305 ┃  67176.786 ┃ 1.180 ┃ B/op  ┃
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━┻━━━━━┻━━━━━━━━━━┻━━━━━━━┻━━━━━━━━━━━┻━━━━━━━┻━━━━━━━━━━━┻━━━━━━━━┻━━━━━━━━━━━━┻━━━━━━━┻━━━━━━━┛
```

  - Avoid lambda allocation in getOrElseUpdate call
  - Avoid intermediate Tuple2 instances when iterating through
    the mutable map.
  - Avoid allocating Map1-4 during building the result if
    the size of the grouped exceeds 4. Intead build a HashMap
    directly.

```
./benchdb list --limit 2 && ./benchdb results --run 5 --run 6 --pivot size  && ./benchdb results --run 5 --run 6 --pivot size --metric "·gc.alloc.rate.norm"
┏━━━━┳━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━┓
┃ ID ┃ Timestamp           ┃ Msg              ┃
┣━━━━╋━━━━━━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━━━┫
┃  6 ┃ 2020-08-24 07:14:36 ┃ PR 8948          ┃
┃  5 ┃ 2020-08-24 07:06:47 ┃ PR 8948 Baseline ┃
┗━━━━┻━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━━━┛
2 test runs found (limit reached).
┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━┳━━━━━┳━━━━━━━━━━┳━━━━━━━━━┳━━━━━━━━━━━┳━━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━┓
┃                                                (size) ┃                ┃                ┃      ┃     ┃        128         ┃         512         ┃         2048          ┃         8192          ┃       ┃
┃ Benchmark                                             ┃ (hashCodeCost) ┃ (maxNumGroups) ┃ Mode ┃ Cnt ┃    Score ┃   Error ┃     Score ┃   Error ┃      Score ┃    Error ┃      Score ┃    Error ┃ Units ┃
┣━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━╋━━━━━╋━━━━━━━━━━╋━━━━━━━━━╋━━━━━━━━━━━╋━━━━━━━━━╋━━━━━━━━━━━━╋━━━━━━━━━━╋━━━━━━━━━━━━╋━━━━━━━━━━╋━━━━━━━┫
┃ scala/scala#5:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 8631.963 ┃ 116.790 ┃ 30969.939 ┃ 456.711 ┃ 119820.013 ┃ 1741.671 ┃ 480813.306 ┃ 7268.723 ┃ ns/op ┃
┃ scala/scala#6:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 8287.717 ┃ 135.038 ┃ 30672.551 ┃ 540.409 ┃ 119356.530 ┃ 1592.624 ┃ 477570.346 ┃ 6365.010 ┃ ns/op ┃
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━┻━━━━━┻━━━━━━━━━━┻━━━━━━━━━┻━━━━━━━━━━━┻━━━━━━━━━┻━━━━━━━━━━━━┻━━━━━━━━━━┻━━━━━━━━━━━━┻━━━━━━━━━━┻━━━━━━━┛
┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━┳━━━━━┳━━━━━━━━━━┳━━━━━━━┳━━━━━━━━━━━┳━━━━━━━┳━━━━━━━━━━━┳━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━┳━━━━━━━┓
┃                                                (size) ┃                ┃                ┃      ┃     ┃       128        ┃        512        ┃        2048        ┃        8192        ┃       ┃
┃ Benchmark [·gc.alloc.rate.norm]                       ┃ (hashCodeCost) ┃ (maxNumGroups) ┃ Mode ┃ Cnt ┃    Score ┃ Error ┃     Score ┃ Error ┃     Score ┃  Error ┃      Score ┃ Error ┃ Units ┃
┣━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━╋━━━━━╋━━━━━━━━━━╋━━━━━━━╋━━━━━━━━━━━╋━━━━━━━╋━━━━━━━━━━━╋━━━━━━━━╋━━━━━━━━━━━━╋━━━━━━━╋━━━━━━━┫
┃ scala/scala#5:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 2192.014 ┃ 0.022 ┃ 13712.051 ┃ 0.076 ┃ 50949.533 ┃ 56.381 ┃ 198544.803 ┃ 1.213 ┃ B/op  ┃
┃ scala/scala#6:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 1896.014 ┃ 0.021 ┃  5224.051 ┃ 0.077 ┃ 17768.204 ┃  0.305 ┃  67176.786 ┃ 1.180 ┃ B/op  ┃
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━┻━━━━━┻━━━━━━━━━━┻━━━━━━━┻━━━━━━━━━━━┻━━━━━━━┻━━━━━━━━━━━┻━━━━━━━━┻━━━━━━━━━━━━┻━━━━━━━┻━━━━━━━┛
```