[committed] libstdc++: Fix std::variant to reject array types [PR116381]

[Jonathan Wakely]

Tested x86_64-linux. Pushed to trunk.

Probably worth backporting too. It could potentially cause new errors
for people using arrays in std::variant, but that's forbidden by the
standard.

-- >8 --

libstdc++-v3/ChangeLog:

        PR libstdc++/116381
        * include/std/variant (variant): Fix conditions for
        static_assert to match the spec.
        * testsuite/20_util/variant/types_neg.cc: New test.
---
 libstdc++-v3/include/std/variant                |  6 ++----
 .../testsuite/20_util/variant/types_neg.cc      | 17 +++++++++++++++++
 2 files changed, 19 insertions(+), 4 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/20_util/variant/types_neg.cc

diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 12108d07f0b..5fb7770d889 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -1457,10 +1457,8 @@ namespace __detail::__variant
 
       static_assert(sizeof...(_Types) > 0,
                    "variant must have at least one alternative");
-      static_assert(!(std::is_reference_v<_Types> || ...),
-                   "variant must have no reference alternative");
-      static_assert(!(std::is_void_v<_Types> || ...),
-                   "variant must have no void alternative");
+      static_assert(((std::is_object_v<_Types> && !is_array_v<_Types>) && ...),
+                   "variant alternatives must be non-array object types");
 
       using _Base = __detail::__variant::_Variant_base<_Types...>;
 
diff --git a/libstdc++-v3/testsuite/20_util/variant/types_neg.cc 
b/libstdc++-v3/testsuite/20_util/variant/types_neg.cc
new file mode 100644
index 00000000000..5cd3d02154b
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/variant/types_neg.cc
@@ -0,0 +1,17 @@
+// { dg-do compile { target c++17 } }
+
+# include <variant>
+
+std::variant<> v0; // { dg-error "here" }
+// { dg-error "must have at least one alternative" "" { target *-*-* } 0 }
+std::variant<int, void> v1; // { dg-error "here" }
+std::variant<int, const void> v2; // { dg-error "here" }
+std::variant<int, int&> v3; // { dg-error "here" }
+std::variant<int, void()> v4; // { dg-error "here" }
+std::variant<int, int[]> v5; // { dg-error "here" }
+std::variant<int, int[1]> v6; // { dg-error "here" }
+// { dg-error "must be non-array object types" "" { target *-*-* } 0 }
+
+// All of variant's base classes are instantiated before checking any
+// static_assert, so we get lots of errors before the expected errors above.
+// { dg-excess-errors "" }
-- 
2.46.0