[libc++][ranges] Fix incorrect integer type in `view_interface` tests.

[var-const]

Summary

ForwardIter() - ForwardIter() returns ptrdiff_t, and converting it
to an unsigned type isn't guaranteed to produce the same type as
size_t.

[var-const]

libcxx/test/std/ranges/range.utility/view.interface/view.interface.pass.cpp

237–240

Please let me know if you think this line is overkill.

[ldionne]

Thanks!

libcxx/test/std/ranges/range.utility/view.interface/view.interface.pass.cpp

237–240

I would add a comment that says something like // test the test.

libcxx/test/std/ranges/range.utility/view.interface/view.interface.pass.cpp

 
 // template<class D>
 //   requires is_class_v<D> && same_as<D, remove_cv_t<D>>
 // class view_interface;
 
 #include <ranges>
 
 #include <cassert>
+#include <utility>
 #include "test_macros.h"
 #include "test_iterators.h"
 
 template<class T>
 concept ValidViewInterfaceType = requires { typename std::ranges::view_interface<T>; };
 
 struct Empty { };
 

 template<class T>
 concept SizeInvocable = requires (T const& obj) { obj.size(); };
 
 constexpr bool testSize() {
   static_assert(!SizeInvocable<InputRange>);
   static_assert(!SizeInvocable<NotSizedSentinel>);
   static_assert( SizeInvocable<ForwardRange>);
 
-  using SignedSize = std::common_type_t<std::ptrdiff_t, std::make_signed_t<std::size_t>>;
+  // Test the test.
+  static_assert(std::same_as<decltype(std::declval<ForwardIter>() - std::declval<ForwardIter>()), std::ptrdiff_t>);
+  using UnsignedSize = std::make_unsigned_t<std::ptrdiff_t>;
+  using SignedSize = std::common_type_t<std::ptrdiff_t, std::make_signed_t<UnsignedSize>>;
   ForwardRange forwardRange;
   assert(forwardRange.size() == 8);
   assert(static_cast<ForwardRange const&>(forwardRange).size() == 8);
 
   assert(std::ranges::size(forwardRange) == 8);
-  static_assert(std::same_as<decltype(std::ranges::size(std::declval<ForwardRange>())), std::size_t>);
+  static_assert(std::same_as<decltype(std::ranges::size(std::declval<ForwardRange>())), UnsignedSize>);
   static_assert(std::same_as<decltype(std::ranges::ssize(std::declval<ForwardRange>())), SignedSize>);
 
   assert(std::ranges::size(static_cast<ForwardRange const&>(forwardRange)) == 8);
-  static_assert(std::same_as<decltype(std::ranges::size(std::declval<ForwardRange const>())), std::size_t>);
+  static_assert(std::same_as<decltype(std::ranges::size(std::declval<ForwardRange const>())), UnsignedSize>);
   static_assert(std::same_as<decltype(std::ranges::ssize(std::declval<ForwardRange const>())), SignedSize>);
 
   SizeIsTen sizeTen;
   assert(sizeTen.size() == 10);
   assert(static_cast<SizeIsTen const&>(sizeTen).size() == 10);
   assert(sizeTen.std::ranges::view_interface<SizeIsTen>::size() == 8);
 
   assert(std::ranges::size(sizeTen) == 10);