template <typename T>struct A{ static constexpr T obj {}; static constexpr bool noexcept_copy = noexcept( T{obj} ); static void UsesCopy() { T{obj}; } static constexpr int C = 1;};struct NoCopy{ constexpr NoCopy() = default; NoCopy(const NoCopy&) = delete;};int main(){ return A<NoCopy>::C;}The code above is successfully compiled by GCC, but Clang gives a compilation error:
tmp.cpp:6:57: error: call to deleted constructor of 'NoCopy' static constexpr bool noexcept_copy = noexcept( T{obj} ); ^~~~~~tmp.cpp:20:16: note: in instantiation of template class 'A<NoCopy>' requested here return A<NoCopy>::C; ^tmp.cpp:15:9: note: 'NoCopy' has been explicitly marked deleted here NoCopy(const NoCopy&) = delete; ^1 error generated.The A::UsesCopy function uses copy constructor as well, but the compiler does not complain about the usage of deleted function there. What is the difference between UsesCopy function and noexcept_copy constexpr? Both use copy constructor of NoCopy class and both are not used but the constexpr definition produces a compilation error, the function definition does not.
PS. Clang compiles the code above with -std=c++17 or -std=c++2a, but not with -std=c++11 or -std=c++14.