Discards and ref assignment

[GeeLaw]

Describe the bug

Per draft C# 8.0 standard, the statement int i = 0; _ = ref i; is invalid, but Roslyn accepts it. Since the compiler is already out, the standard should be modified to allow ref assignment to discards.

Example

int i = 0;
_ = ref i;

Expected behavior

The compiler should complain.

Actual behavior

The compiler doesn't complain.

Additional context

9.2.9.2 Discards:

A discard is a local variable that has no name. A discard is introduced by a declaration expression (§12.19) with the identifier _; and is either implicitly typed (_ or var _) or explicitly typed (T _).

This means that if _ = ref i; is acceptable, it is a declaration expression.

12.19 Declaration expressions:

... When used as a declaration expression, _ is called a simple discard. It is semantically equivalent to var _, but is permitted in more places.

A declaration expression shall only occur in the following syntactic contexts:

• ...
• As a simple discard _ comprising the left side of a simple assignment (§12.23.2).
• ...

Note "simple assignment".

12.23:

12.23.2 Simple assignment

The = operator is called the simple assignment operator.

12.23.3 Ref assignment

The = ref operator is known as the ref assignment operator.

This means ref assignment is not simple assignment (i.e., ref assignment is not "simple assignment to a variable reference variable", whatever the latter means).

(Memo to self: Once this issue is resolved, depending on how it's resolved, there might be another issue to file. For example, object[] array = ...; _ = ref array[0]; currently compiles to ldelem.ref by Roslyn, instead of ldelema. This difference is important because the latter checks for variance.)

[GeeLaw]

Cross-referencing: discovered when investigating dotnet/roslyn#81574