Championed issue: dotnet/csharplang#3301
Draft spec: dotnet/csharplang#3417

• accessibility modifiers
  • LangVersion checks
  • require implementation part
  • require equivalent accessibility between signature and implementation parts
• 'out' params
  • require accessibility modifiers
• non-'void' returns
  • require accessibility modifiers
• 'virtual', 'override', 'sealed', 'new' modifiers
  • require accessibility modifiers
  • require equivalent sets of modifiers on both parts
• 'extern' modifier
  • require accessibility modifiers
  • only implementation part can specify it
  • when implementation is 'extern' the definition part has IsExtern == true
  • [DllImport] can go on either part
• 'async' modifier
  • require accessibility modifiers (async partial void methods are already allowed)
  • only implementation part can specify it
  • when implementation is 'async' the definition part has 'IsAsync == true' (?)
    • preserving existing behavior that 'IsAsync == false' on definition.
• nullability attributes
  • When overriding a partial method with nullability attributes, the compiler needs to recognize+require providing the same nullability attributes present on both parts unioned.
  • when overriding a partial DIM
• partial methods that implement interface methods
• entry point with non-'void' return
  • with definition and implementation
  • with only a definition
• call to partial method with 'out' parameter definitely assigns the argument
• ref returns
• calls from outside the containing type
  • from a different type
  • from a different compilation
• Override a method when the base is a constructed type Extended partial methods #43582 (comment)
• different containing type kinds
• SkipLocalsInitAttribute on definition or implementation
• EnumeratorCancellationAttribute on definition or implementation
• include modopt from overridden method https://github.com/333fred/roslyn/blob/test/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenFunctionPointersTests.cs#L5269-L5310
• overridden method has an explicit array size https://github.com/333fred/roslyn/blob/test/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenFunctionPointersTests.cs#L6215-L6481
• 'in' thunk (explicit implementation of interface method due to 'in' param)
• 'virtual partial' in interface
• consume extended partial in VB
• XML docs

IDE side:

• go to implementation (it's not clear what is meant by this--F12 on partial methods in general will go to the implementation part if it exists, and go to implementation will go to the interface part. Since this is a pre-existing behavior for partial methods in the IDE I don't think any work related this belongs in this test plan.)
• completion for partials Signature completion for extended partial methods #46752

Chunks of the work:

• Most diagnostic checks in Unexpected warning CS0649: Field '(T1, T2).F1' is never assigned to #43588

  • accessibility mods
  • 'out' params
  • extended modifiers (excluding 'async')

• IsExtern/IsAsync behavior

• Pending decision: we should consider allowing or even requiring repetition of nullability attributes across the definition and implementation parts of the method.

  • (rigibson note) I'm dropping this idea after some consideration. 😄 Can revise the behavior later since it is nullable related.