First off, I note that this is a duplicate of:
Why is Func<T> ambiguous with Func<IEnumerable<T>>?
What's the exact problem here?
Thomas's guess is essentially correct. Here are the exact details.
Let's go through it a step at a time. We have an invocation:
"test".Select<char, Tuple<char>>(Tuple.Create);
Overload resolution must determine the meaning of the call to Select. There is no method "Select" on string or any base class of string, so this must be an extension method.
There are a number of possible extension methods for the candidate set because string is convertible to IEnumerable<char> and presumably there is a using System.Linq; in there somewhere. There are many extension methods that match the pattern "Select, generic arity two, takes an IEnumerable<char> as the first argument when constructed with the given method type arguments".
In particular, two of the candidates are:
Enumerable.Select<char,Tuple<char>>(IEnumerable<char>,Func<char,Tuple<char>>)
Enumerable.Select<char,Tuple<char>>(IEnumerable<char>,Func<char,int,Tuple<char>>)
Now, the first question we face is are the candidates applicable? That is, is there an implicit conversion from each supplied argument to the corresponding formal parameter type?
An excellent question. Clearly the first argument will be the "receiver", a string, and it will be implicitly convertible to IEnumerable<char>. The question now is whether the second argument, the method group "Tuple.Create", is implicitly convertible to formal parameter types Func<char,Tuple<char>>, and Func<char,int, Tuple<char>>.
When is a method group convertible to a given delegate type? A method group is convertible to a delegate type when overload resolution would have succeeded given arguments of the same types as the delegate's formal parameter types.
That is, M is convertible to Func<A, R> if overload resolution on a call of the form M(someA) would have succeeded, given an expression 'someA' of type 'A'.
Would overload resolution have succeeded on a call to Tuple.Create(someChar)? Yes; overload resolution would have chosen Tuple.Create<char>(char).
Would overload resolution have succeeded on a call to Tuple.Create(someChar, someInt)? Yes, overload resolution would have chosen Tuple.Create<char,int>(char, int).
Since in both cases overload resolution would have succeeded, the method group is convertible to both delegate types. The fact that the return type of one of the methods would not have matched the return type of the delegate is irrelevant; overload resolution does not succeed or fail based on return type analysis.
One might reasonably say that convertibility from method groups to delegate types ought to succeed or fail based on return type analysis, but that's not how the language is specified; the language is specified to use overload resolution as the test for method group conversion, and I think that's a reasonable choice.
Therefore we have two applicable candidates. Is there any way that we can decide which is better than the other? The spec states that the conversion to the more specific type is better; if you have
void M(string s) {}
void M(object o) {}
...
M(null);
then overload resolution chooses the string version because string is more specific than object. Is one of those delegate types more specific than the other? No. Neither is more specific than the other. (This is a simplification of the better-conversion rules; there are actually lots of tiebreakers, but none of them apply here.)
Therefore there is no basis to prefer one over the other.
Again, one could reasonably say that sure, there is a basis, namely, that one of those conversions would produce a delegate return type mismatch error and one of them would not. Again, though, the language is specified to reason about betterness by considering the relationships between the formal parameter types, and not about whether the conversion you've chosen will eventually result in an error.
Since there is no basis upon which to prefer one over the other, this is an ambiguity error.
It is easy to construct similar ambiguity errors. For example:
void M(Func<int, int> f){}
void M(Expression<Func<int, int>> ex) {}
...
M(x=>Q(++x));
That's ambiguous. Even though it is illegal to have a ++ inside an expression tree, the convertibility logic does not consider whether the body of a lambda has something inside it that would be illegal in an expression tree. The conversion logic just makes sure that the types check out, and they do. Given that, there's no reason to prefer one of the M's over the other, so this is an ambiguity.
You note that
"test".Select<char, Tuple<char>>(Tuple.Create<char>);
succeeds. You now know why. Overload resolution must determine if
Tuple.Create<char>(someChar)
or
Tuple.Create<char>(someChar, someInt)
would succeed. Since the first one does and the second one does not, the second candidate is inapplicable and eliminated, and is therefore not around to become ambiguous.
You also note that
"test".Select<char, Tuple<char>>(x=>Tuple.Create(x));
is unambiguous. Lambda conversions do take into account the compatibility of the returned expression's type with the target delegate's return type. It is unfortunate that method groups and lambda expressions use two subtly different algorithms for determining convertibility, but we're stuck with it now. Remember, method group conversions have been in the language a lot longer than lambda conversions; had they been added at the same time, I imagine that their rules would have been made consistent.
