libstdc++: Enforce requirements on template argument of std::optional

The standard does not allow std::optional<T&>, std::optional<T[1]>,
std::optional<T()> etc. and although we do give errors, they come from
down inside the internals of std::optional. We could improve the static
assertions at the top of the class so that users get a more precise
diagnostic:

optional:721:21: error: static assertion failed
721 |       static_assert(is_object_v<_Tp> && !is_array_v<_Tp>);

libstdc++-v3/ChangeLog:

	* include/std/optional (optional): Adjust static assertion to
	reject arrays and functions as well as references.
	* testsuite/20_util/optional/requirements_neg.cc: New test.

2 files changed, 25 insertions, 1 deletions

diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional
index 62ff87a..90bf741 100644
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -718,7 +718,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     {
       static_assert(!is_same_v<remove_cv_t<_Tp>, nullopt_t>);
       static_assert(!is_same_v<remove_cv_t<_Tp>, in_place_t>);
-      static_assert(!is_reference_v<_Tp>);
+      static_assert(is_object_v<_Tp> && !is_array_v<_Tp>);
 
     private:
       using _Base = _Optional_base<_Tp>;
diff --git a/libstdc++-v3/testsuite/20_util/optional/requirements_neg.cc b/libstdc++-v3/testsuite/20_util/optional/requirements_neg.cc
new file mode 100644
index 0000000..688c305
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/optional/requirements_neg.cc
@@ -0,0 +1,24 @@
+// { dg-do compile { target c++17 } }
+
+#include <optional>
+
+// T shall be a type other than cv in_place_t or cv nullopt_t
+// that meets the Cpp17Destructible requirements
+
+std::optional<std::nullopt_t> o1;        // { dg-error "here" }
+std::optional<const std::nullopt_t> o2;  // { dg-error "here" }
+std::optional<std::in_place_t> o3;       // { dg-error "here" }
+std::optional<const std::in_place_t> o4; // { dg-error "here" }
+std::optional<int&> o5;                  // { dg-error "here" }
+std::optional<int[1]> o6;                // { dg-error "here" }
+std::optional<int[]> o7;                 // { dg-error "here" }
+std::optional<int()> o8;                 // { dg-error "here" }
+
+// { dg-error "static assertion failed" "" { target *-*-* } 0 }
+
+// { dg-prune-output "forming pointer to reference type" }
+// { dg-prune-output "union may not have reference type" }
+// { dg-prune-output "function returning an array" }
+// { dg-prune-output "flexible array member .* in union" }
+// { dg-prune-output "function returning a function" }
+// { dg-prune-output "invalidly declared function type" }