No conversion from old-style anonymous method to expression tree

[gafter]

I found this in the Roslyn test sources:

            // The spec has an omission; I believe we intended it to say that there is no
            // conversion from any old-style anonymous method expression to any expression tree
            // type. This is the rule the native compiler enforces; Roslyn should as well, and 
            // we should clarify the specification.

with this test case

using System;
using System.Linq.Expressions;
class Program
{
    static void Main()
    {
        Foo(delegate { }); // No error; chooses the non-expression version.
    }
    static void Foo(Action a) { }
    static void Foo(Expression<Action> a) { }
}

[gafter]

We are now taking language feature discussion on https://github.com/dotnet/csharplang for C# specific issues, https://github.com/dotnet/vblang for VB-specific features, and https://github.com/dotnet/csharplang for features that affect both languages.

See also #18002 for a general discussion of closing and/or moving language issues out of this repo.