Extended Description

// https://godbolt.org/z/mAL4Sd
#include
int a();
int b();
int main() {
std::midpoint(a, b);
}

====

/c++/v1/numeric:554:52: error: arithmetic on pointers to the function type 'int ()'
return __a + _VSTD::midpoint(ptrdiff_t(0), __b - __a);
~~~ ^ ~~~

According to N4810, std::midpoint for T* should be constrained to require T to be a complete object type (not a function type, and not cv void either).

I'm fairly confident that no library code should ever try to constrain on is_pointer without also asking whether it's an object pointer, function pointer, member pointer, or void pointer. It might be worth auditing the entire libc++ codebase for uses of is_pointer.