Under `-Ydelambdafy:method`, a public, static accessor method is
created to expose the private method containing the body of the
lambda.

Currently this accessor method has its parameters in the same order
structure as those of the lambda body method.

What is this order? There are three categories of parameters:

  1. lambda parameters
  2. captured parameters (added by lambdalift)
  3. self parameters (added to lambda bodies that end up in trait
     impl classes by mixin, and added unconditionally to the static
     accessor method.)

These are currently emitted in order #3, #1, #2.

Here are examples of the current behaviour:

BEFORE (trait):

```
% cat sandbox/test.scala && scalac-hash v2.11.5 -Ydelambdafy:method sandbox/test.scala && javap -private -classpath . 'Test$class'
trait Member; class Capture; trait LambdaParam
trait Test {
  def member: Member
  def foo {
    val local = new Capture
    (arg: LambdaParam) => "" + arg + member + local
  }
}
Compiled from "test.scala"
public abstract class Test$class {
  public static void foo(Test);
  private static final java.lang.String $anonfun$1(Test, LambdaParam, Capture);
  public static void $init$(Test);
  public static final java.lang.String accessor$1(Test, LambdaParam, Capture);
}
```

BEFORE (class):

```
% cat sandbox/test.scala && scalac-hash v2.11.5 -Ydelambdafy:method sandbox/test.scala && javap -private -classpath . Test
trait Member; class Capture; trait LambdaParam
abstract class Test {
  def member: Member
  def foo {
    val local = new Capture
    (arg: LambdaParam) => "" + arg + member + local
  }
}
Compiled from "test.scala"
public abstract class Test {
  public abstract Member member();
  public void foo();
  private final java.lang.String $anonfun$1(LambdaParam, Capture);
  public Test();
  public static final java.lang.String accessor$1(Test, LambdaParam, Capture);
}
```

Contrasting the class case with Java:

```
% cat sandbox/Test.java && javac -d . sandbox/Test.java && javap -private -classpath . Test
public abstract class Test {
  public static class Member {};
  public static class Capture {};
  public static class LambaParam {};
  public static interface I {
    public abstract Object c(LambaParam arg);
  }
  public abstract Member member();
  public void test() {
    Capture local = new Capture();
    I i1 = (LambaParam arg) -> "" + member() + local;
  }
}

Compiled from "Test.java"
public abstract class Test {
  public Test();
  public abstract Test$Member member();
  public void test();
  private java.lang.Object lambda$test$0(Test$Capture, Test$LambaParam);
}
```

We can see that in Java 8 lambda parameters come after captures. If we
want to use Java's LambdaMetafactory to spin up our anoymous FunctionN
subclasses on the fly, our ordering must change.

I can see three options for change:

  1. Adjust `LambdaLift` to always prepend captured parameters,
     rather than appending them. I think we could leave `Mixin` as
     it is, it already prepends the self parameter. This would result
     a parameter ordering, in terms of the list above: #3, #2, #1.
  2. More conservatively, do this just for methods known to hold
     lambda bodies. This might avoid needlessly breaking code that
     has come to depend on our binary encoding.
  3. Adjust the parameters of the accessor method only. The body
     of this method can permute params before calling the lambda
     body method.

This commit implements option #2.

In also prototyped #1, and found it worked so long as I limited it to
non-constructors, to sidestep the need to make corresponding
changes elsewhere in the compiler to avoid the crasher shown
in the enclosed test case, which was minimized from a bootstrap
failure from an earlier a version of this patch.

We would need to defer option #1 to 2.12 in any case, as some of
these lifted methods are publicied by the optimizer, and we must
leave the signatures alone to comply with MiMa.

I've included a test that shows this in all in action. However, that
is currently disabled, as we don't have a partest category for tests
that require Java 8.

The log messages intented to chronicle implicit search were
always being filtered out by virtue of the fact that the the tree
passed to `printTyping` was already typed, (e.g. with an implicit
MethodType.)

This commit enabled printing in this case, although it still
filters out trees that are deemed unfit for typer tracing,
such as `()`. In the context of implicit search, this happens
to filter out the noise of:

```
|    |    |    [search #2] start `()`, searching for adaptation to pt=Unit => Foo[Int,Int] (silent: value <local Test> in Test) implicits disabled
|    |    |    [search #3] start `()`, searching for adaptation to pt=(=> Unit) => Foo[Int,Int] (silent: value <local Test> in Test) implicits disabled
|    |    |    \-> <error>
```

... which I think is desirable.

The motivation for this fix was to better display the interaction
between implicit search and type inference. For instance:

```
class Foo[A, B]
class Test {
  implicit val f: Foo[Int, String] = ???
  def t[A, B](a: A)(implicit f: Foo[A, B]) = ???
  t(1)
}
```

````
% scalac -Ytyper-debug sandbox/instantiate.scala
...
|    |-- t(1) BYVALmode-EXPRmode (site: value <local Test> in Test)
|    |    |-- t BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local Test> in Test)
|    |    |    [adapt] [A, B](a: A)(implicit f: Foo[A,B])Nothing adapted to [A, B](a: A)(implicit f: Foo[A,B])Nothing
|    |    |    \-> (a: A)(implicit f: Foo[A,B])Nothing
|    |    |-- 1 BYVALmode-EXPRmode-POLYmode (site: value <local Test> in Test)
|    |    |    \-> Int(1)
|    |    solving for (A: ?A, B: ?B)
|    |    solving for (B: ?B)
|    |    [search #1] start `[A, B](a: A)(implicit f: Foo[A,B])Nothing` inferring type B, searching for adaptation to pt=Foo[Int,B] (silent: value <local Test> in Test) implicits disabled
|    |    [search #1] considering f
|    |    [adapt] f adapted to => Foo[Int,String] based on pt Foo[Int,B]
|    |    [search #1] solve tvars=?B, tvars.constr= >: String <: String
|    |    solving for (B: ?B)
|    |    [search #1] success inferred value of type Foo[Int,=?String] is SearchResult(Test.this.f, TreeTypeSubstituter(List(type B),List(String)))
|    |    |-- [A, B](a: A)(implicit f: Foo[A,B])Nothing BYVALmode-EXPRmode (site: value <local Test> in Test)
|    |    |    \-> Nothing
|    |    [adapt] [A, B](a: A)(implicit f: Foo[A,B])Nothing adapted to [A, B](a: A)(implicit f: Foo[A,B])Nothing
|    |    \-> Nothing
```

Top level modules in Scala currently desugar as:

```
class C; object O extends C { toString }
```

```
public final class O$ extends C {
  public static final O$ MODULE$;

  public static {};
    Code:
       0: new           #2                  // class O$
       3: invokespecial scala#12                 // Method "<init>":()V
       6: return

  private O$();
    Code:
       0: aload_0
       1: invokespecial scala#13                 // Method C."<init>":()V
       4: aload_0
       5: putstatic     scala#15                 // Field MODULE$:LO$;
       8: aload_0
       9: invokevirtual scala#21                 // Method java/lang/Object.toString:()Ljava/lang/String;
      12: pop
      13: return
}
```

The static initalizer `<clinit>` calls the constructor `<init>`, which
invokes superclass constructor, assigns `MODULE$= this`, and then runs
the remainder of the object's constructor (`toString` in the example
above.)

It turns out that this relies on a bug in the JVM's verifier: assignment to a
static final must occur lexically within the <clinit>, not from within `<init>`
(even if the latter is happens to be called by the former).

I'd like to move the assignment to <clinit> but that would
change behaviour of "benign" cyclic references between modules.

Example:

```
package p1; class CC { def foo = O.bar}; object O {new CC().foo; def bar = println(1)};

// Exiting paste mode, now interpreting.

scala> p1.O
1
```

This relies on the way that we assign MODULE$ field after the super class constructors
are finished, but before the rest of the module constructor is called.

Instead, this commit removes the ACC_FINAL bit from the field. It actually wasn't
behaving as final at all, precisely the issue that the stricter verifier
now alerts us to.

```
scala> :paste -raw
// Entering paste mode (ctrl-D to finish)

package p1; object O

// Exiting paste mode, now interpreting.

scala> val O1 = p1.O
O1: p1.O.type = p1.O$@ee7d9f1

scala> scala.reflect.ensureAccessible(p1.O.getClass.getDeclaredConstructor()).newInstance()
res0: p1.O.type = p1.O$@64cee07

scala> O1 eq p1.O
res1: Boolean = false
```

We will still achieve safe publication of the assignment to other threads
by virtue of the fact that `<clinit>` is executed within the scope of
an initlization lock, as specified by:

  https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-5.html#jvms-5.5

Fixes scala/scala-dev#SD-194

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 scala#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 scala#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 scala#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)
                ^
```

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 #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 #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 #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 #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.

# This is the 1st commit message:

Optimize BitSet#min and max for case of Ordering.Int

# This is the commit message #2:

Fix buggy bitset min and max implementations