Member functions of class templates not instantiated despite `[[gnu::used]]`

[llvmbot]

Bugzilla Link	19923
Version	3.4
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor

Extended Description

In a class template, g++ instantiates member functions marked with [[gnu::used]] even when those functions are used nowhere in the program. clang does not. This causes code to compile that doesn't compile on g++, e.g.

#include <type_traits>

template<class I>
class X {
  [[gnu::used]]
  static void check() {
    static_assert(std::is_enum<I>::value, "");
  }
};

X<int> x;

[llvmbot]

mentioned in issue llvm/llvm-bugzilla-archive#46059