C# 7.3 method_body etc. grammar does not allow => ref

[KalleOlaviNiemitalo]

Describe the bug

In C# 7.3, a method/property/accessor/indexer body => ref variable_reference; should be allowed and mean the same as { return ref variable_reference; }. Roslyn allows this, but the grammar in the C# 7 draft doesn't.

This issue concerns multiple clauses, listed under "Expected behavior" below.

Example

The following should be allowed, but => ref field does not match the syntactic grammar.

class C {
    int field;
    ref int ShortProperty => ref field;
    ref int LongProperty { get => ref field; }
    ref int this[int index] => ref field;
    ref int Method() => ref field;
    void Outer() {
        ref int Inner() => ref field;
    }
}

Expected behavior

In §15.7.1 (Classes / Properties / General), property_body should allow => ref variable_reference ;.

In §15.7.3 (Classes / Properties / Accessors), accessor_body should allow => ref variable_reference ;.

In §15.9 (Classes / Indexers), indexer_body should allow => ref variable_reference ;.

In §15.6.1 (Classes / Methods / General), method_body should allow => ref variable_reference ;.

In §13.6.4 (Statements / Declaration statements / Local function declarations), local_function_body should allow => ref variable_reference ;.

The standard should specify what the => ref syntax means, perhaps by reference to §13.10.5 (Statements / Jump statements / The return statement), which specifies the meaning of return ref.

Additional context

In §12.19.1 (Expressions / Anonymous function expressions / General), anonymous_function_body already allows ref variable_reference. However, I haven't found anything that specifies what ref means in this context.

In §15.10.1 (Classes / Operators / General), operator_body does not allow => ref variable_reference ; but this is OK as operators cannot return by reference; operator_declarator does not allow ref either.

In §15.11.1 (Classes / Instance constructors / General), constructor_body does not allow => ref variable_reference ; but this is OK as instance constructors cannot return values either.

In §15.12 (Classes / Static constructors), static_constructor_body does not allow => ref variable_reference ; but this is OK as static constructors cannot return values either.

In §15.13 (Classes / Finalizers), finalizer_body does not allow => ref variable_reference ; but this is OK as finalizers cannot return values either.

Elaborated from #213 (comment)

[jskeet]

@Nigel-Ecma: If you've got bandwidth to look at this, that would be great - if not, let me know and I'll see if @gafter can.

[KalleOlaviNiemitalo]

The specification of this feature should be at https://github.com/dotnet/csharplang/blob/cd336064a26a43c31c1164ef7cd3f5feb4420d20/proposals/csharp-7.0/expression-bodied-everything.md or https://github.com/dotnet/csharplang/blob/cd336064a26a43c31c1164ef7cd3f5feb4420d20/proposals/csharp-7.0/ref-locals-returns.md but both are only placeholders.

[Nigel-Ecma]

Covered by PR #837