Clang will ignore using declarations from qualified name lookups when handling declarators

[ChuanqiXu9]

See https://godbolt.org/z/8eef5nsEs and the text form is:

namespace lib {
    namespace impl {
        template <class>
        inline constexpr bool test = false;
    }
    using impl::test;
}

struct foo {};

template <>
inline constexpr bool lib::test<foo> = true;

static_assert(lib::test<foo>);

GCC will accept this but Clang will reject this.

[llvmbot]

@llvm/issue-subscribers-c-1

[ChuanqiXu9]

I sent https://reviews.llvm.org/D148506 as a ad-hoc patch.

[Endilll]

I think GCC behaves correctly, and Clang doesn't:

0. test on line 12 is a qualified name (basicl.lookup.qual/2.4.5).
1. test has to undergo a qualified name lookup in the lookup context of lib from the point where it appears (basicl.lookup.qual/3).
2. Qualified name lookup is a search in associated scope (basicl.lookup.qual/3).
3. Search is a single search, because lib is not a class or class template (class.member.lookup/1).
4. Single search finds all declarations in lib that precede the point where test appears, taking into account using declarations whose terminal name is not dependent (basic.lookup.general/3).
5. Name test on line 6 is not dependent (temp.dep.type/5), which means it should be found.

Most of this wording comes from P1787R6. I hope this helps!

[llvmbot]

@llvm/issue-subscribers-clang-frontend