Test plan for "using aliases for any types"

[jcouv]

Championed proposal: dotnet/csharplang#4284
Speclet: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-12.0/using-alias-types.md

Note this doesn't include support for generic aliases (draft spec dotnet/csharplang#4452)

Compiler

• LangVer (tests with RegularNext or CSharpNext)
• spec: https://github.com/dotnet/csharplang/blob/main/proposals/using-alias-types.md
  • Consider documenting rule that unsafe aliases are also checked at usage point
• LDM: using unsafe or unsafe using? LDM decision 2/1/2023. using unsafe. This keeps all the using forms consistent that they start with using (or global using).
• Various types:
  • tuples (need positive test)
  • built-in types: int, nint, ...
  • array types
  • pointer and function pointer
  • ref type (StaticUsingDirectiveRefType)
  • dynamic
  • nullable reference types (disallowed: using X = object?;, using X = Y?; where Y is another alias) (AliasUsingNullableReferenceType)
• unsafe requires proper compilation flag
• using unsafe in a non-alias using (see UsingUnsafeNonAlias)
• global alias (with and without unsafe) (see GlobalAliasToType...)
• duplicate alias (ERR_DuplicateAlias)
• semantic model: GetTypeInfo on the RHS, IAliasSymbol.Target, GetAliasInfo
• SyntaxNormalizer (using unsafe x = ..., using x = (one, two);, ...)
• Obsolete: using X = (ObsoleteType, ObsoleteType);, using X = ObsoleteType[];
• EnC: Yes. Needs tests. See TopLevelEditingTests.Using_Global_Insert
• using T = dynamic; where a dynamic type already exists. AliasUsingDirectiveDynamic4 and AliasUsingDirectiveDynamic4.
• using unsafe X = int; then use X in safe context (no diagnostic). TestUnsafeAlias14_A, TestUnsafeAlias14_B
• using X = scoped R;. TestScopedType1-4
• using X = ref readonly R; AliasUsingDirectiveRefReadonlyType
• using X = __makeref(...);. TestMakeRef
• using X = __arglist();. TestArgList
• review public API changes. Done: API proposal for "Using Alias to any Type" #66913
• inspect the syntactic pointer type detection logic in compiler (is there a circularity issue?). Have made the change to no longer do things syntactically. Added tests to ensure no issue (TestStructWithReferenceToItselfThroughAliasPointer1/2). But need eyes on this.
• debugger support (using X = dynamic; inspect a value of type X). Added in Update PDB behavior for 'using alias to type', and add EE and PDB tests. #67105
• add an EE test. Added in Update PDB behavior for 'using alias to type', and add EE and PDB tests. #67105.
• LDM: should we allow unsafe on using static? (using static C<int*[]>; is already legal, warning wave?)

Productivity

• Completion
• Completion on dynamic
• Classification
• QuickInfo
• Formatting
• Rename tracking

[tmat]

Note this will need a design and updated spec of custom debug information for PDBs - at least for Portable PDBs. We can decide to not make this available in EEs when Windows PDBs are used. If we support Windows PDBs the CDIs need to be designed for Windows PDBs as well. The format for Windows PDBs is different than Portable.