[libc++] Shrink variant's index type when possible

Summary:
Currently `std::variant` always uses an unsigned int to store the variant index. However this isn't nessesary and causes `std::variant` to be larger than it needs to be in most cases.

This patch changes the index type to be `unsigned char` when possible, and `unsigned short` or `unsigned int` otherwise, depending on the size (Although it's questionable if it's even possible to create a variant with 65535 elements.

Unfortunately this change is an ABI break, and as such is only enabled in ABI v2.

Reviewers: mpark

Reviewed By: mpark

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D40210

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@318621 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/utilities/variant/variant.variant/variant_size.pass.cpp

@@ -0,0 +1,68 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+// <variant>
+
+// template <class ...Types> class variant;
+
+#include <limits>
+#include <type_traits>
+#include <utility>
+#include <variant>
+
+template <class Sequence>
+struct make_variant_imp;
+
+template <size_t ...Indices>
+struct make_variant_imp<std::integer_sequence<size_t, Indices...>> {
+  using type = std::variant<decltype((Indices, char(0)))...>;
+};
+
+template <size_t N>
+using make_variant_t = typename make_variant_imp<std::make_index_sequence<N>>::type;
+
+constexpr bool ExpectEqual =
+#ifdef _LIBCPP_ABI_VARIANT_INDEX_TYPE_OPTIMIZATION
+  false;
+#else
+  true;
+#endif
+
+template <class IndexType>
+void test_index_type() {
+  using Lim = std::numeric_limits<IndexType>;
+  using T1 = make_variant_t<Lim::max() - 1>;
+  using T2 = make_variant_t<Lim::max()>;
+  static_assert((sizeof(T1) == sizeof(T2)) == ExpectEqual, "");
+}
+
+template <class IndexType>
+void test_index_internals() {
+  using Lim = std::numeric_limits<IndexType>;
+  static_assert(std::__choose_index_type(Lim::max() -1) !=
+                std::__choose_index_type(Lim::max()), "");
+  static_assert(std::is_same_v<
+      std::__variant_index_t<Lim::max()-1>,
+      std::__variant_index_t<Lim::max()>
+    > == ExpectEqual, "");
+  using IndexT = std::__variant_index_t<Lim::max()-1>;
+  using IndexLim = std::numeric_limits<IndexT>;
+  static_assert(std::__variant_npos<IndexT> == IndexLim::max(), "");
+}
+
+int main() {
+  test_index_type<unsigned char>();
+  // This won't compile due to template depth issues.
+  //test_index_type<unsigned short>();
+  test_index_internals<unsigned char>();
+  test_index_internals<unsigned short>();
+}