c++ - static_assert dependent on non-type template parameter (different behavior on gcc and clang)

Question

template <int answer> struct Hitchhiker {
  static_assert(sizeof(answer) != sizeof(answer), "Invalid answer");
};

template <> struct Hitchhiker<42> {};

While trying to disable general template instantiation with static_assert I discovered that the above code in clang generates the assert error even when the template is not instantiated, while gcc generates the assert error only when instantiating Hitchhiker with a parameter other than 42.

Fiddling around I found that this assert:

template <int answer> struct Hitchhiker {
  static_assert(sizeof(int[answer]) != sizeof(int[answer]), "Invalid answer");
};

template <> struct Hitchhiker<42> {};

behaves the same on both compilers: the assert kicks in only when the general template is instantiated.

What does the standard says, which compiler is right?

g++ 4.9.2
clang++ 3.50

Answer 1

Both compilers are correct. From [temp.res]/8:

If no valid specialization can be generated for a template, and that template is not instantiated, the template is ill-formed, no diagnostic required.

There does not exist a valid specialization that can be generated from the primary template Hitchhiker, so it is ill-formed, no diagnostic required. clang chooses to issue a diagnostic anyway.

If you only want to allow 42, then simply don't define the general template:

template <int > struct Hitchhiker;
template <> struct Hitchhiker<42> {};