Test plan for "async method builder override"

[jcouv]

Proposal: dotnet/csharplang#1407
Spec: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-10.0/async-method-builders.md

• Usage with type parameter: [MyBuilderOverride<int>] (currently disallowed) (clarifying message in PR Use specific error message for wrong arity in async builder #54632)
• Update attribute type in BCL (issue API Proposal: AsyncMethodBuilderOverrideAttribute and PoolingAsyncValueTaskMethodBuilders runtime#49903, PR Add AsyncMethodBuilderOverride and PoolingAsyncValueTaskMethodBuilders runtime#50116)
• LDM confirm we need a fully functional task-like return type (answer: unused so unvalidated, but lambdas must have explicit return types)
• LDM confirm we keep the same validation rule on builder type (answer: no accessibility requirement)
• LDM confirm we keep the same validation rules on well-known members (answer: all must be public)
• LDM confirm that the builder factory vs. builder indirection only applies to overrides (answer: indirection is now out-of-scope)
• LangVer 😉
• No validation on builder of return type. The return type may not even be task-like.
  • We also test with Task/ValueTask return types (that’s one of the primary scenarios)
  • Nullability analysis of return statement
• Binding rules for return statement:
  • return with no value and implicit return allowed if method-level builder is non-generic
  • return with value involves conversion to the type argument
• Attribute can be used on method, lambda with explicit type, and local function
• The type used as method-level builder must be a non-void non-error named type.
  • We validate the builder the same way that we do for task-like types, except that we don’t care about the method-level builder’s accessibility.
• We validate numerous kinds of bad builders: missing members, incorrect signatures or return types, static, ref return, non-public accessibility, …
• Specifying the attribute more than once (first one is used)
• No IDE impact

Task-like types spec: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-7.0/task-types.md

[jcouv]

Feature was merged for dev17.0p2