[libc++] Fix ability to explicitly instantiate std::midpoint (#74217)

std::midpoint is specified by having a pointer overload in
[numeric.ops.midpoint].
With the way the pointer overload is specified, users can expect that
calling
std::midpoint as `std::midpoint<T>(a, b)` should work, but it didn't in
libc++
due to the way the pointer overload was specified.

Fixes #67046

libcxx/include/__numeric/midpoint.h

@@ -48,12 +48,8 @@ midpoint(_Tp __a, _Tp __b) noexcept _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK
   return __a + __half_diff;
 }
 
-template <class _TPtr>
-_LIBCPP_HIDE_FROM_ABI constexpr enable_if_t<
-    is_pointer_v<_TPtr> && is_object_v<remove_pointer_t<_TPtr>> && !is_void_v<remove_pointer_t<_TPtr>> &&
-        (sizeof(remove_pointer_t<_TPtr>) > 0),
-    _TPtr>
-midpoint(_TPtr __a, _TPtr __b) noexcept {
+template <class _Tp, enable_if_t<is_object_v<_Tp> && !is_void_v<_Tp> && (sizeof(_Tp) > 0), int> = 0>
+_LIBCPP_HIDE_FROM_ABI constexpr _Tp* midpoint(_Tp* __a, _Tp* __b) noexcept {
   return __a + std::midpoint(ptrdiff_t(0), __b - __a);
 }
 

libcxx/test/libcxx/numerics/numeric.ops/midpoint.integer.pass.cpp

@@ -22,14 +22,14 @@
 
 //  Users are not supposed to provide template argument lists for
 //  functions in the standard library (there's an exception for min and max)
-//  However, libc++ protects against this for pointers, so we check to make
-//  sure that our protection is working here.
-//  In some cases midpoint<int>(0,0) might get deduced as the pointer overload.
+//  However, libc++ protects against this for pointers. The use of T(0)
+//  in the test cases resolves potential ambiguity in template argument deduction
+//  for the std::midpoint function.
 
 template <typename T>
 void test()
 {
-    ASSERT_SAME_TYPE(T, decltype(std::midpoint<T>(0, 0)));
+  ASSERT_SAME_TYPE(T, decltype(std::midpoint<T>(T(0), T(0))));
 }
 
 int main(int, char**)

libcxx/test/std/numerics/numeric.ops/numeric.ops.midpoint/midpoint.pointer.pass.cpp

@@ -54,6 +54,12 @@ void runtime_test()
     assert(std::midpoint(array +    9, array) == array + 5);
     assert(std::midpoint(array +   10, array) == array + 5);
     assert(std::midpoint(array +   11, array) == array + 6);
+
+    // explicit instantiation
+    ASSERT_SAME_TYPE(decltype(std::midpoint<T>(array, array)), T*);
+    ASSERT_NOEXCEPT(std::midpoint<T>(array, array));
+    assert(std::midpoint<T>(array, array) == array);
+    assert(std::midpoint<T>(array, array + 1000) == array + 500);
 }
 
 template <typename T>