Ambiguity accessing identifier in global and nested anonymous namespaces

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The following code:

namespace { constexpr int x = 1; }
namespace X { namespace { constexpr int x = 2; } }
using namespace X;

int main() {
  static_assert( ::x == 1 );
}

successfully compiles in MSVC, which prefers the first anonymous namespace in finding of ::x over X::(anonymous namespace).

However both GCC and Clang reject the code due to ambiguity of selection. Demo: https://gcc.godbolt.org/z/zPEGzGar3

Which compiler is right here?

language-lawyer namespaces c++
1 Answers

There is a similar example in this reference of unnamed namespaces:

namespace {
    int i; // defines ::(unique)::i
}
void f() {
    i++;   // increments ::(unique)::i
}
 
namespace A {
    namespace {
        int i;        // A::(unique)::i
        int j;        // A::(unique)::j
    }
    void g() { i++; } // A::(unique)::i++
}
 
using namespace A; // introduces all names from A into global namespace
void h() {
    i++;    // error: ::(unique)::i and ::A::(unique)::i are both in scope
    A::i++; // ok, increments ::A::(unique)::i
    j++;    // ok, increments ::A::(unique)::j
}

According to this, rejecting the code due to ambiguity of selection by GCC and Clang should be correct.

Namespaces provide a method for preventing name conflicts in large projects.

Symbols declared inside a namespace block are placed in a named scope that prevents them from being mistaken for identically-named symbols in other scopes.

As this explanation states, namespace is introduced to avoid or solve the name conflicts. IMHO these code examples are obviously trying to make name conflicts.

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