[Bug c++/82247] [concepts] Name deduction in concepts fails depending on the argument type

["Casey at Carter dot net" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82247

Casey Carter <Casey at Carter dot net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |Casey at Carter dot net

--- Comment #4 from Casey Carter <Casey at Carter dot net> ---
This is not a bug: it's two-phase name lookup. At the point of definition for
Fooable:

#include <string>
#include <vector>

template <typename T>
bool concept Fooable = requires(const T& t) {
    { foo(t) }; // foo is unknown here
};

the compiler records the set of declarations it has seen for "foo" (the empty
set). When you later "instantiate" the concept foo can only resolve to (a) a
member of the (again, empty) set recorded at definition, or (b) a declaration
found by argument-dependent lookup. Since there are no associated namespaces
for fundamental types, only your user-defined type which has a "foo" defined in
its associated namespace successfully concept checks.