由于命名空间，c++ 无法解释的类“尚未声明”错误

Question

I have some template class that has two private static members. Users define a traits struct and provide it to the template class, which then derives from it.

Then in a c++ file the user define the static members, with one member initialized from the other. For some reason I get a "class has not been declared" error if I dont fully specify the namespace for the arg. This is only an issue when I'm in a nested namespace, there is no issue if you define the type in a single top level namespace, which makes me think this is a compiler bug. Trimmed down example below, compiling with gcc 7.2

template<typename Traits>
struct Base 
{
    static int x;
    static int y;
};

namespace foo::bar
{
    struct BarTraits
    {
    };

    using Bar = Base<BarTraits>;

    template<> int Bar::x = 0;
    template<> int Bar::y( Bar::x );  //error 
    //template<> int Bar::y( foo::bar::Bar::x ); //no error
}

Answer 1

According to C++ standard temp.expl.spec#3

An explicit specialization may be declared in any scope in which the corresponding primary template may be defined.

Your code violates this statement, because Bar::x and Bar::y are specialized in namespace foo::bar .

GCC incorrectly accepts first two specializations because of known defect https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56119

The following fixed code

template<typename Traits>
struct Base  {
    static int x;
    static int y;
};

struct BarTraits {};
using Bar = Base<BarTraits>;

template<> int Bar::x = 0;
template<> int Bar::y( Bar::x );

is accepted by GCC, Clang, MSVC: https://gcc.godbolt.org/z/MPxjTzbah