libstdc++: Fix tests that fail in C++20 mode

	* testsuite/20_util/is_constructible/51185.cc: Make test class a
	non-aggregate so that the test verifies the same thing in all -std
	modes.
	* testsuite/20_util/is_constructible/value-2.cc: Adjust expected
	results for some types when paren-init for aggregates is supported.

libstdc++-v3/ChangeLog

 2020-04-20  Jonathan Wakely  <jwakely@redhat.com>
 
+	* testsuite/20_util/is_constructible/51185.cc: Make test class a
+	non-aggregate so that the test verifies the same thing in all -std
+	modes.
+	* testsuite/20_util/is_constructible/value-2.cc: Adjust expected
+	results for some types when paren-init for aggregates is supported.
+
 	* include/std/version (__cpp_lib_three_way_comparison): Update value.
 	* libsupc++/compare (__cpp_lib_three_way_comparison): Likewise.
 	(__detail::__synth3way): Add noexcept-specifier.

libstdc++-v3/testsuite/20_util/is_constructible/51185.cc

@@ -20,7 +20,14 @@
 #include <type_traits>
 
 struct A { };
-struct B : A { };
+struct B : A {
+#if __cpp_aggregate_bases && __cpp_aggregate_paren_init
+  // A user-declared constructor prevents B from being an aggregate.
+  // Otherwise 'B&& b{A{}};' becomes valid in C++17 (__cpp_aggregate_bases),
+  // and 'B&& b(A{});' becomes valid in C++17 (__cpp_aggregate_paren_init).
+  B();
+#endif
+};
 
 // libstdc++/51185
 void f()

libstdc++-v3/testsuite/20_util/is_constructible/value-2.cc

@@ -216,12 +216,20 @@ static_assert(std::is_constructible<const B&&, D&&>::value, "Error");
 static_assert(!std::is_constructible<B&, const D&>::value, "Error");
 static_assert(!std::is_constructible<B&&, const D&&>::value, "Error");
 
+#if __cpp_aggregate_bases && __cpp_aggregate_paren_init
+// In C++20 an rvalue reference or const lvalue reference can bind to a
+// temporary of aggregate type that is initialized from a base class value.
+constexpr bool v = true;
+#else
+constexpr bool v = false;
+#endif
+
 static_assert(!std::is_constructible<D&, B&>::value, "Error");
-static_assert(!std::is_constructible<D&&, B&&>::value, "Error");
+static_assert(v == std::is_constructible<D&&, B&&>::value, "Error");
 static_assert(!std::is_constructible<D&, const B&>::value, "Error");
-static_assert(!std::is_constructible<D&&, const B&&>::value, "Error");
-static_assert(!std::is_constructible<const D&, B&>::value, "Error");
-static_assert(!std::is_constructible<const D&&, B&&>::value, "Error");
+static_assert(v == std::is_constructible<D&&, const B&&>::value, "Error");
+static_assert(v == std::is_constructible<const D&, B&>::value, "Error");
+static_assert(v == std::is_constructible<const D&&, B&&>::value, "Error");
 
 static_assert(!std::is_constructible<B&&, B&>::value, "Error");
 static_assert(!std::is_constructible<B&&, D&>::value, "Error");
@@ -754,14 +762,21 @@ static_assert(!std::is_constructible<FromArgs<std::initializer_list<int>&,
 	      std::initializer_list<B>&>, std::initializer_list<int>,
 	      std::initializer_list<B>>::value, "Error");
 
+#if __cpp_aggregate_paren_init
+// In C++20 arrays can be initialized using parentheses.
+constexpr bool w = true;
+#else
+constexpr bool w = false;
+#endif
+
 static_assert(!std::is_constructible<FromArgs<std::initializer_list<int>>,
 	      int, int>::value, "Error");
 static_assert(!std::is_constructible<const
 	      FromArgs<std::initializer_list<int>>, int, int>::value, "Error");
-static_assert(!std::is_constructible<B[2], B, B>::value, "Error");
-static_assert(!std::is_constructible<const B[2], B, B>::value, "Error");
-static_assert(!std::is_constructible<U[2], U, U>::value, "Error");
-static_assert(!std::is_constructible<const U[2], U, U>::value, "Error");
+static_assert(w == std::is_constructible<B[2], B, B>::value, "Error");
+static_assert(w == std::is_constructible<const B[2], B, B>::value, "Error");
+static_assert(w == std::is_constructible<U[2], U, U>::value, "Error");
+static_assert(w == std::is_constructible<const U[2], U, U>::value, "Error");
 
 static_assert(!std::is_constructible<E, E, E>::value, "Error");
 static_assert(!std::is_constructible<const E, E, E>::value, "Error");