Problem

Let's say I have the following extension method:

public Func<T2, TResult> Curry<T1, T2, TResult>(this Action<T1, T2, TResult> action, T1 arg) {}

and the following class method:

public static int Add(int x1, int x2);

If I call this extension method as a normal method, it looks like this:

Curry(Add, 5);

However as an extension method, it would look like this:

((Action<int, int>)Add).Curry(5);

Another related problem is with interpolated strings -- since extension methods do not affect type choice, it is not possible to call IFormattable extension methods on $"".

Potential solution

I suggest allowing extension methods to influence the type choice for the method group.
Example:

Add.Curry(5);

Similar logic should apply to $"".

Pros

1. Consistency between different call types
2. Reduced verbosity in functional scenarios (e.g. Select(Add.Curry(5)))
3. Allows extension methods on FormattableString

Cons

1. Resolution is pretty complex as it is, not sure if there are any edge cases that would complicate this further