Allow creating open delegates from instance method groups

[tmat]

Currently C# does not allow to create an open delegate given an method group that has a matching instance method overload like so:

new Func<string, string, bool>(string.Contains)

error CS0123: No overload for 'Contains' matches delegate 'Func<string, string, bool>'

The Reflection API allows such delegate to be created using:

typeof(string).GetMethod("Contains").CreateDelegate(typeof(Func<string, string, bool>))

The following should also work:

new Func<string, string>(string.ToLowerInvariant)

The reflection equivalent works:

typeof(string).GetMethod("ToLowerInvariant").CreateDelegate(typeof(Func<string, string>));

[huoyaoyuan]

The valid syntax with same semantics should be (x, y) => x.Contains(y). It will require an inlining, the open delegate version will require an IL stub to move the ABI, so I'd expect their codegen to be idential.

[tmat]

I'd also like this to work:

new Func<string, string>(string.ToLowerInvariant)

The reflection equivalent works:

typeof(string).GetMethod("ToLowerInvariant").CreateDelegate(typeof(Func<string, string>));

[Updated the original text.]

[tmat]

@huoyaoyuan Yes, you can write lambda but that's extra clutter. You'd probably want to use static to ensure its closure-free: static (x, y) => x.Contains(y). It's better to just type string.Contains.