[libc++] Add a test for std::ssize's SFINAE

Inspired by https://reviews.llvm.org/D120684#inline-1157644 and
subsequent LWG discussion. See http://wg21.link/LWG3207 for
additional context.

Differential Revision: https://reviews.llvm.org/D121154

Co-authored-by: Louis Dionne <ldionne.2@gmail.com>

libcxx/test/std/iterators/iterator.container/ssize.LWG3207.compile.pass.cpp

@@ -0,0 +1,37 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// <iterator>
+
+// template <class T, ptrdiff_t N>
+//   constexpr ptrdiff_t ssize(const T (&array)[N]) noexcept;
+//
+// This test checks that the library implements LWG3207 as not-a-defect.
+// `clang -m32` is an example of a configuration where using ptrdiff_t
+// instead of size_t in std::ssize has an observable SFINAE effect.
+//
+// REQUIRES: 32-bit-pointer
+
+#include <iterator>
+#include <climits>
+#include <cstddef>
+
+// Test the test:
+static_assert(sizeof(std::ptrdiff_t) == 4, "Run only on these platforms");
+static_assert(sizeof(std::size_t) == 4, "Run only on these platforms");
+static_assert(std::size_t(PTRDIFF_MAX) + 1 > std::size_t(PTRDIFF_MAX), "This should always be true");
+extern char forming_this_type_must_be_valid_on_this_platform[std::size_t(PTRDIFF_MAX) + 1];
+
+// The actual test:
+template <class T>
+concept HasSsize = requires(T&& t) { std::ssize(t); };
+
+static_assert(HasSsize<char[std::size_t(PTRDIFF_MAX)]>);
+static_assert(!HasSsize<char[std::size_t(PTRDIFF_MAX) + 1]>);