specification of the foreach statement (12.9.5) doesn't apply for System.Span

[ltcmelo]

The foreach statement is specified by means of an expansion into a while *.
But the specification doesn't apply for a type such as System.Span or System.ReadOnlySpan as the expansion will produce invalid code.
Specifically: the V in foreach (V v in x) «embedded_statement» will expand into the V in V v = (V)(T)e.Current;; however, if the foreach is inside an async method, then the declaration V v will lead to a diagnose such as:

error CS4012: Parameters or locals of type 'ReadOnlySpan<char>' cannot be declared in async methods or async lambda expressions.

Here's an example in which specified expansion breaks.

public class C
{
    public async Task M()
    {
        IReadOnlyList<string> o = null;
        foreach (ReadOnlySpan<char> c in o) {}        
        return;
    }
}

* I have (most of) this expansion as a syntax rewriter.

[CyrusNajmabadi]

@ltcmelo It's unclear what you're asking for here.

[ltcmelo]

I suppose that I need (to click on the button) "Create issue from discussion" then.

[CyrusNajmabadi]

I'm reporting a bug in the specification.

I don't understand what the bug is that you're reporting. Can you expand on this?

[BillWagner]

@ltcmelo

C# specification (standard) issues should be reported in the dotnet/csharpstandard repo. That's where the committee is doing its work. This will be a timely issue. The committee is working on the updates for C# 7.x and we would have likely missed that ref struct declarations would impact this area of the language. (Its less about foreach, but more relate to any use of a ref struct in an async method.

[ltcmelo]

OK, will do that then. Thanks.

[ltcmelo]

I was told that the root of the issue is Roslyn accepting the original program, and that I should open a report in Roslyn. Here it is: dotnet/roslyn#62747