Championed issue: dotnet/csharplang#6051
Spec: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-12.0/lambda-method-group-defaults.md

General

• Lang Version
• Sync with partner teams on method group breaking change
• Missing constant value types
• Compile BCL
• Compile VS
• GetSymbolInfo returns delegate type with default parameters
• GetConstantValue on default parameter value
• IOperation symbols reflect default values

Syntax

• Parse parameter initializers
  • Delegate syntax
  • Lambda syntax
• Missing expression after =
  • Delegate syntax
  • Lambda syntax
• No parentheses around the parameter for lambda
• Missing parameter type
  • Delegate syntax
  • Lambda syntax
• With attributes
• arglist with initializer
• (int i = > 0) => { }

Semantics

• Valid constant values
  • sbyte
  • byte
  • short
  • ushort
  • int
  • uint
  • long
  • ulong
  • nint
  • nuint
  • char
  • float
  • double
  • decimal
  • bool
  • enum
  • string
  • null reference
• Target-typed constant conversion
  • Valid literal conversion
  • Valid non-literal conversion
• Non-constant expression
  • Non-constant non-literal
  • Non-constant literal (u8 string, interpolated string)
  • Interpolated string handler conversion
• Invalid constant conversion
• Recursive parameter reference
  • Parameter before the reference
  • Parameter after the reference
• Constant value from the same scope
  • Defined before the lambda
  • Defined after the lambda
• Well-known attributes that affect constant parameters
  • DefaultValueAttribute
  • CallerInfoX
  • Params
• unsafe scenarios
  • sizeof
  • int* parameter with null default value
• Non-default parameters trailing
• Mismatched default parameters between lambda and target delegate type
  • Both have different constant values - Error
  • Target delegate has constant value, lambda does not - Valid
  • Target delegate does not have constant value, lambda does - Error
• Mismatched default parameters between method group and target delegate type
  • Both have different constant values - Warning
  • Target delegate has constant value, method does not - Valid
  • Target delegate does not have constant value, method does - Valid
• Conversion between different delegate types is unaffected
• var produces anonymous delegate type with default parameters and does not use Action/Func
  • Lambda
  • Method group
• Nullability analysis
• Discard parameters with defaults.
• Does (dynamic d = null) => { } create a delegate with dynamic parameter type? (Same question for (dynamic d, ref int i) => { }.)
• Do (T t = default) => { }, (T t = null) => { } share a delegate type when T is appropriately constrained?
• MemberSemanticModel should be parented to the correct outer SemanticModel
  • symbols for locals declared outside of a lambda referenced inside default value should be reference identical to symbols returned by GetDeclaredSymbol for the locals and for references to the locals outside of the lambda
  • binder chain should be shared with member model for the enclosing method body
  • step through the code to make sure the correct things happen
• SpeculativeSemanticModel to ensure errors from default parameter values are not added to declaration table
• Expression trees: follow up with LDM on whether it should be allowed
• new[] { (int i = 1) => { }, (int i = 2) => { } } is there a best common type or an error?
• Matching default value to generic type:

  delegate D<T>(T t = default);
  void M<T>(D<T> p) { }
  M((object o = null) => {});

• NoPIA: can an embedded type leak through to the delegate type? (not specific to default params)
• A parameter with a ref, out or this modifier cannot have a default_argument
• A parameter_array may occur after an optional parameter, but cannot have a default value

Emit

• Emits an anonymous delegate type with default parameters
• Generices non-default parameters where possible
• Shares lambda definitions where default parameters are the same
• Emitted Invoke method for the lambda has default parameters

Productivity

• FindAllReferences
• Unused constants in default parameter values
• Parameter default value completion when typing
• Extract local uses var
• Convert to method preserves default values
• EnC: default lambda parameters #69246