11.8 Method group conversions requires error even if the conversion isn't used

[gafter]

Consider this little test program

public class Program1
{
    delegate void MyAction<T>(T x);
 
    void Y(long x) { }
 
    void D(MyAction<int> o, short s) { }
    void D(MyAction<long> o, int i) { }
 
    void T()
    {
        D(Y, 1);
    }
}

The first argument in the call D(Y, 1) is a method group. During overload resolution, we will seek the existence of a method group conversion to see which methods are applicable.

There is an implicit conversion from argument Y to the first parameter of the first D method, according to 11.8:

• A conversion is considered to exist if the algorithm of §12.7.6.2 produces a single best method M having the same number of parameters as D.
• Even if the conversion exists, a compile-time error occurs if the selected method M is not compatible (§20.4) with the delegate type D.

There certainly is a best method, as there is only one Y. But since Y is not compatible with the delegate type MyAction<int>, a compile-time error occurs. This is even though the compiler would not even try to apply that conversion, instead selecting the other overload of D as the one to be called!

Actual compilers do not report this compile-time error unless the conversion is attempted to be used. This test program compiles and runs.

[jskeet]

This looks like it's worth doing, and may be reasonably small. It's possible that it's already fixed in a later version of C# (as overload resolution has definitely changed).

[jskeet]

(Applied the "gnarly" label on general principle, as it's to do with overload resolution.)

[jskeet]

Note to self: see if #263 fixes this.