Non-constexpr specialisation declaration following constexpr declaration

Question

The C++ program below compiles with GCC trunk:

template <typename T>
struct Foo
{
  constexpr void test();
};

void not_constexpr() {}

template <>
constexpr void Foo<int>::test() { }

template <>
          void Foo<float>::test() { not_constexpr(); }

int main() {}

But fails with Clang and MSVC. The error message is:

<source>:13:28: error: non-constexpr declaration of 'test' follows constexpr declaration
   13 |           void Foo<float>::test() { not_constexpr(); }
      |                            ^
<source>:4:18: note: previous declaration is here
    4 |   constexpr void test();

This can be seen on Compiler Explorer here. Which compiler is correct?

Answer 1

GCC is correct: inline, constexpr, constinit, consteval, attributes, and contracts are independent for explicit specializations ([temp.expl.spec]/12). The situation is complicated by the special feature of specializing a member of a class template rather than a function template directly (which Clang supports), but that doesn’t make the previous declaration any more controlling.