How to get the return type of a member function from within a class?

Question

The following program yields a compilation error with clang, though it passes on other compilers:

#include <utility>

struct foo
{
  auto bar() -> decltype(0)
  {
    return 0;
  }

  using bar_type = decltype(std::declval<foo>().bar());
};

int main()
{
  return 0;
}

clang yields:

$ clang -std=c++11 clang_repro.cpp 
clang_repro.cpp:10:48: error: member access into incomplete type 'foo'
  using bar_type = decltype(std::declval<foo>().bar());
                                               ^
clang_repro.cpp:3:8: note: definition of 'foo' is not complete until the closing '}'
struct foo
       ^
1 error generated.

Is this program illegal, and if so, is there a correct way to define foo::bar_type?

clang details:

$ clang --version
Ubuntu clang version 3.5-1ubuntu1 (trunk) (based on LLVM 3.5)
Target: x86_64-pc-linux-gnu
Thread model: posix

Answer 1

g++4.9 issues the same error

I'm not sure if this is an invalid code, because incomplete types are allowed for declval, and expression in decltype is not evaluated.
rightføld in his answer explained very good why this code is invalid.

You can use std::result_of:

using bar_type = std::result_of<decltype(&foo::bar)(foo)>::type;

Which is actually implemented like this:

using bar_type = decltype((std::declval<foo>().*std::declval<decltype(&foo::bar)>())());

The difference between this and the code in the question is that pointer-to-member operator (.*) is used instead of member access operator (.), and it doesn't require the type to be complete, which is demonstrated by this code:

#include <utility>
struct foo;
int main() {
    int (foo::*pbar)();
    using bar_type = decltype((std::declval<foo>().*pbar)());
}