[libcxx] Use a type that is always an aggregate in variant's tests

[ldionne]

Summary

In PR39232, we noticed that some variant tests started failing in C++2a mode
with recent Clangs, because the rules for literal types changed in C++2a. As
a result, a temporary fix was checked in (enabling the test only in C++17).

This commit is what I believe should be the long term fix: I removed the
tests that checked constexpr default-constructibility with a weird type
from the tests for index() and valueless_by_exception(), and instead I
added tests for those using an obviously literal type in the test for the
default constructor.

[mclow.lists]

LGTM

libcxx/trunk/test/std/utilities/variant/variant.variant/variant.ctor/default.pass.cpp

 #include <cassert>
 #include <type_traits>
 #include <variant>
 
 #include "test_macros.h"
 #include "variant_test_helpers.hpp"
 
 struct NonDefaultConstructible {
-  NonDefaultConstructible(int) {}
+  constexpr NonDefaultConstructible(int) {}
 };
 
 struct NotNoexcept {
   NotNoexcept() noexcept(false) {}
 };
 
 #ifndef TEST_HAS_NO_EXCEPTIONS
 struct DefaultCtorThrows {
void test_default_ctor_basic() {
     assert(std::get<0>(v) == 0);
   }
   {
     std::variant<int, long> v;
     assert(v.index() == 0);
     assert(std::get<0>(v) == 0);
   }
   {
+    std::variant<int, NonDefaultConstructible> v;
+    assert(v.index() == 0);
+    assert(std::get<0>(v) == 0);
+  }
+  {
     using V = std::variant<int, long>;
     constexpr V v;
     static_assert(v.index() == 0, "");
     static_assert(std::get<0>(v) == 0, "");
   }
   {
     using V = std::variant<int, long>;
     constexpr V v;
     static_assert(v.index() == 0, "");
     static_assert(std::get<0>(v) == 0, "");
   }
+  {
+    using V = std::variant<int, NonDefaultConstructible>;
+    constexpr V v;
+    static_assert(v.index() == 0, "");
+    static_assert(std::get<0>(v) == 0, "");
+  }
 }
 
 int main() {
   test_default_ctor_basic();
   test_default_ctor_sfinae();
   test_default_ctor_noexcept();
   test_default_ctor_throws();
 }

libcxx/trunk/test/std/utilities/variant/variant.variant/variant.status/index.pass.cpp

 // -*- 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
 
-// The following compilers don't consider a type an aggregate type (and
-// consequently not a literal type) if it has a base class at all.
-// In C++17, an aggregate type is allowed to have a base class if it's not
-// virtual, private, nor protected (e.g. ConstexprTestTypes:::NoCtors).
-// XFAIL: gcc-5, gcc-6
-// XFAIL: clang-3.5, clang-3.6, clang-3.7, clang-3.8
-// XFAIL: apple-clang-6, apple-clang-7, apple-clang-8.0
-
 // <variant>
 
 // template <class ...Types> class variant;
 
 // constexpr size_t index() const noexcept;
 
 #include <cassert>
 #include <string>
 #include <type_traits>
 #include <variant>
 
 #include "archetypes.hpp"
 #include "test_macros.h"
 #include "variant_test_helpers.hpp"
 
+
 int main() {
-#if TEST_STD_VER == 17
-  { // This test does not pass on C++20 or later; see https://bugs.llvm.org/show_bug.cgi?id=39232
-    using V = std::variant<int, ConstexprTestTypes::NoCtors>;
+  {
+    using V = std::variant<int, long>;
     constexpr V v;
     static_assert(v.index() == 0, "");
   }
-#endif
+  {
+    using V = std::variant<int, long>;
+    V v;
+    assert(v.index() == 0);
+  }
   {
     using V = std::variant<int, long>;
     constexpr V v(std::in_place_index<1>);
     static_assert(v.index() == 1, "");
   }
   {
     using V = std::variant<int, std::string>;
     V v("abc");

libcxx/trunk/test/std/utilities/variant/variant.variant/variant.status/valueless_by_exception.pass.cpp

 // -*- 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
 
-// The following compilers don't consider a type an aggregate type (and
-// consequently not a literal type) if it has a base class at all.
-// In C++17, an aggregate type is allowed to have a base class if it's not
-// virtual, private, nor protected (e.g. ConstexprTestTypes:::NoCtors).
-// XFAIL: gcc-5, gcc-6
-// XFAIL: clang-3.5, clang-3.6, clang-3.7, clang-3.8
-// XFAIL: apple-clang-6, apple-clang-7, apple-clang-8.0
-
 // <variant>
 
 // template <class ...Types> class variant;
 
 // constexpr bool valueless_by_exception() const noexcept;
 
 #include <cassert>
 #include <string>
 #include <type_traits>
 #include <variant>
 
 #include "archetypes.hpp"
 #include "test_macros.h"
 #include "variant_test_helpers.hpp"
 
+
 int main() {
-#if TEST_STD_VER == 17
-  { // This test does not pass on C++20 or later; see https://bugs.llvm.org/show_bug.cgi?id=39232
-    using V = std::variant<int, ConstexprTestTypes::NoCtors>;
+  {
+    using V = std::variant<int, long>;
     constexpr V v;
     static_assert(!v.valueless_by_exception(), "");
   }
-#endif
+  {
+    using V = std::variant<int, long>;
+    V v;
+    assert(!v.valueless_by_exception());
+  }
   {
     using V = std::variant<int, long, std::string>;
     const V v("abc");
     assert(!v.valueless_by_exception());
   }
 #ifndef TEST_HAS_NO_EXCEPTIONS
   {
     using V = std::variant<int, MakeEmptyT>;
     V v;
     assert(!v.valueless_by_exception());
     makeEmpty(v);
     assert(v.valueless_by_exception());
   }
 #endif
 }