An example code from the standard (templates and concepts) does not compile

[mmatrosov]

Bugzilla Link	50864
Version	trunk
OS	Linux
CC	@erichkeane,@zygoloid,@viccie30,@jwakely

Extended Description

According to [temp.constr.decl]/4 the following example should compile:

template <class T> concept C = true;
template <class T> struct A {
  template <class U> U f(U) requires C<typename T::type>;   // #1
  template <class U> U f(U) requires C<T>;                  // #2
};

template <> template <class U>
U A<int>::f(U u) requires C<int> { return u; }              // OK, specializes #2

However, clang gives an error:

<source>:3:49: error: type 'int' cannot be used prior to '::' because it has no members
  template <class U> U f(U) requires C<typename T::type>;   // #1
                                                ^

I believe this is the reason why clang cannot compile libstdc++ ranges: #46091

[erichkeane]

I'm almost positive this is a duplicate of #44178 #c4.

The problem here is that we're greedily evaluating concepts immediately upon instantiation of the class, rather than waiting until they are called.

[jwakely]

I'm almost positive this is a duplicate of
#44178 #c4.

Agreed. I was about to say that Bug 47509 (which links here) is a dup of that one.

[royjacobson]

Still doesn't work after https://reviews.llvm.org/D126907 but I think it's a different bug now.

<source>:8:11: error: out-of-line definition of 'f' does not match any declaration in 'A<int>'
U A<int>::f(U u) requires C<int> { return u; }              // OK, specializes #2
          ^

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[royjacobson]

Sema::isOverload compares the concepts of the definition to the uninstantiated constraints of the initial declaration, and they're differnt. So Sema::CheckOverload in Sema::CheckFunctionDeclaration thinks this is an overload and not a definition.

I think we need to instantiate the specialized struct and then do a lookup or something like that, I'm not sure how this is only a problem for constrained methods so I probably missed something.

[usx95]

The left over bug is same as #49620.