SFINAE: checking the existence of a function breaks when the overload is moved to other namespaces

Question

I want to check for the existence of a function in a specific namespace using SFINAE. I have found SFINAE to test a free function from another namespace which does the job,

Answer 1

In addition to DyP's answer and following his comment:

If your function bar took any arguments, you could make use of dependent name lookup to make it work (w/o a second overload of bar).

Indeed in my real code bar() does take arguments.

As a side question, is there any way to achieve correct SFINAE detection without polluting the global namespace...

So yes, dependent name lookup works like a charm. For the sake of completeness, and in case it can help others in the future, here's my now perfectly working code:

#define ENABLE_FOO_BAR 1

namespace foo {
  #if ENABLE_FOO_BAR
    int bar(int);
  #endif
}

namespace feature_test {
  namespace detail {
    using namespace foo;
    template decltype(bar(std::declval())) test(int);
    template void test(...);
  }
  static constexpr bool has_foo_bar = std::is_same(0)), int>::value;
  static_assert(has_foo_bar == ENABLE_FOO_BAR, "something went wrong");
}

All credit goes to DyP, I don't believe I'd have thought about this by myself.