C# 7.x: ref locals and returns

[RexJaeschke]

This (Draft) PR is not quite complete.

[RexJaeschke]

Completed edits. Removed Draft status.

[KalleOlaviNiemitalo]

Missing:

• => ref in method_body and similar. However, the draft-v7 branch does not even allow => there yet; C# 6.0 feature: expression bodied members #4 was merged to draft-v6 but those changes are not in draft-v7. C# 7.x: Adding more support for expression-bodied members #69 does not add them, either.

• ref return type in property_declaration and indexer_declaration. Roslyn allows these, and at least System.Span<T> and System.Span<T>.Enumerator have public members like that.

• ref return type in operator_declaration. AFAIK, Roslyn does not allow this, so perhaps you'll decide to leave this out, even though I don't foresee any specification problem in allowing this in unary_operator_declarator and binary_operator_declarator. In contrast, conversion_operator_declarator converting to a reference seems like it would require changes elsewhere in the standard.

[KalleOlaviNiemitalo]

As I wrote in #169 (comment), the standard should describe the runtime array element type checks required for ref locals and returns. Moreover, it should describe how those checks are skipped when ref readonly is used. See also #264 (comment) regarding type checks for in parameters.

[333fred]

One thing I don't see in this spec, @RexJaeschke, is how to handle a case like dotnet/roslyn#53113 (forgive me if it's already subsumed by other parts of the spec; if it is, I couldn't find what does). Based on how we handle M(ref *(int*)0), it seems like such code should be allowed and should not dereference the pointer, but I didn't see anything in the ref locals section talking about it.

[BillWagner]

Converted to draft status (C# 7 feature)

[KalleOlaviNiemitalo]

If #104 is merged before this, I suppose this will need to add => ref for local functions, too.

[gafter]

This still needs to safety rules to be incorporated. See #334 and https://github.com/dotnet/csharplang/blob/standard-proposals/proposals/csharp-7.2/span-safety.md#draft-language-specification . Those safety rules are for ref return, ref locals, and ref structs.