MethodGroup causes ambiguity over the same lambda expression

[JackHerring]

Given the following interface:

public interface IBuilder
{
    void Property( Func<ValueTask<string>> source );
    void Property( Func<Task<string>> source );
}

The following does not compile:

class Program
{
    static void Main( string[] args )
    {
        IBuilder builder = null;

        builder.Property( Source );
    }

    private static ValueTask<string> Source()
    {
        return new ValueTask<string>();
    }
}

However, if you pass a lambda expression which just invokes Source then it compiles fine:

builder.Property( () => Source() );

Why is the first call (using method grouping) failing? A bug..?

[JackHerring]

FYI have raised an issue @jrote1

[DavidArno]

This appears unrelated to Task and ValueTask and will occur for any two types, eg:

public interface IBuilder
{
    void Property( Func<string> source );
    void Property( Func<int> source );
}

class Program
{
    static void Main( string[] args )
    {
        IBuilder builder = null;

        builder.Property( Source ); // ambiguous call
    }

    private static int Source()
    {
        return 0;
    }
}

Method groups are resolved using the arity and types of the parameters, not the return type. So presumably since you are using Func<T>, and thus there are no parameters, method group resolution can't be applied here?

[ufcpp]

#98

For a method group conversion, candidate methods whose return type doesn't match up with the delegate's return type are removed from the set.