ADL does not work in constexpr functions (clang only)

Question

The following code compiles with MSVC and gcc, but not with clang. Why is that so?

It seems like if ADL would not work if CallFoo () is constexpr. See the comment.

template <class T>
constexpr void CallFoo  ()          // Remove constexpr to fix clang compilation error.
{
    Foo (T ());
}


class Apple {};


int main ()
{
    CallFoo<Apple> ();
}


constexpr void Foo (Apple)
{
}

Clang error message (see on godbolt.org):

<source>:4:5: error: use of undeclared identifier 'Foo'
    Foo (T ());
    ^
<source>:13:5: note: in instantiation of function template specialization 'CallFoo<Apple>' requested here
    CallFoo<Apple> ();
    ^

Intuitional

Clang enforces the rule that functions must be declared before they are instantiated by a template, move your function declaration higher up and it'll probably work – Zacheryzack

Neither of the three compilers enforce this. If you remove constexpr in the commented line, all thee compile it successfully. – Intuitional

An extremely bizarre observation! – Fleabite

Answer 1

Declaration should be visible at the point of instantiation, so clang has right to reject your code. Reordering functions fixes compilation:

constexpr void Foo (Apple)
{
}

int main ()
{
    CallFoo<Apple> ();
}

Demo

Fact is that end of file is a point of instantiation too, and gcc/MSVC should only consider this one :/

Balladmonger

Neither of the three compilers enforce that declarations should be visible at the point of instantiation. If you remove constexpr in the commented line, all thee compile it successfully. – Intuitional