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

[ldionne]

Summary

Inspired by https://reviews.llvm.org/D120684#inline-1157644 and subsequent LWG discussion.
See https://cplusplus.github.io/LWG/issue3207 for not-enough-background. :)

Wondering how to restrict this to run on just these weird 32-bit-ptrdiff_t/64-bit-pointer platforms, without running the risk of quietly nerfing the test forever and never noticing.

@ldionne, I wonder if there's a way to get buildkite to alert us about test files that have been skipped in all jobs. That would be generally useful.

[• Quuxplusone]

Try a .sh.cpp test. This might be ready for review, although I assume I'll have to UNSUPPORTED a few platforms depending on what CI says.

[ldionne]

[Github PR transition cleanup]

Picking this up, I think the test is relevant.

[ldionne]

Rebase, make into a .compile.pass.cpp test.

[ldionne]

Don't hardcode -m32, instead only run the test on 32 bit platforms.

[ldionne]

Poke CI.

[ldionne]

CI passed, phab report step is broken.

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

+//===----------------------------------------------------------------------===//
+//
+// 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]>);