[libcxx] MSVC does not provide constexpr for typeid and typeid is probably not constexpr

[danlark]

Summary

Actually, there is nothing about constexpr qualifier for typeid operator in the standard and MSVC is probably correct in this case.

I can be wrong and if you convince me that typeid should be constexpr, I would discard this review and submit a bug to the Microsoft compiler team

[EricWF]

MSVC is wrong and this code is well formed. Please file a bug with MSVC.
The wording is somewhere in [expr.const].

The value of the typeid object isn't usable in a constant expression, but the address of the typeid object is. Similarly.

void test() {
  static int x = rand();
  constexpr int& y = x; // OK
 static_assert(&y); // OK
 static_assert(y); // BOOM!;
}

The constexpr here is important to ensuring that the __policy_ variable has a constant initializer and therefore doesn't need or emit a guard variable.

include/functional

struct __policy
     _LIBCPP_INLINE_VISIBILITY static const __policy* __create()
     {
         return __choose_policy<_Fun>(__use_small_storage<_Fun>());
     }
 
     _LIBCPP_INLINE_VISIBILITY
     static const __policy* __create_empty()
     {
-        static const _LIBCPP_CONSTEXPR __policy __policy_ = {nullptr, nullptr,
-                                                             true,
+        static const __policy __policy_ = {nullptr, nullptr,
 #ifndef _LIBCPP_NO_RTTI
-                                                             &typeid(void)
 #else
-                                                             nullptr
 #endif
         };
         return &__policy_;
     }
 
   private:
     template <typename _Fun> static void* __large_clone(const void* __s)
     {