libstdc++: Add static_assert to std::integer_sequence [PR112473]

C++20 allows class types as non-type template parameters, but
std::integer_sequence explicitly disallows them. Enforce that.

libstdc++-v3/ChangeLog:

	PR libstdc++/112473
	* include/bits/utility.h (integer_sequence): Add static_assert.
	* testsuite/20_util/integer_sequence/112473.cc: New test.

(cherry picked from commit 0953497a81)

libstdc++-v3/include/bits/utility.h

@@ -163,6 +163,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Tp, _Tp... _Idx>
     struct integer_sequence
     {
+#if __cplusplus >= 202002L
+      static_assert(is_integral_v<_Tp>);
+#endif
       typedef _Tp value_type;
       static constexpr size_t size() noexcept { return sizeof...(_Idx); }
     };

libstdc++-v3/testsuite/20_util/integer_sequence/112473.cc

@@ -0,0 +1,8 @@
+// { dg-do compile { target c++20 } }
+
+// PR libstdc++/112473 - integer_sequence accepts non-integer types
+
+#include <utility>
+
+std::integer_sequence<std::pair<int, int>, std::pair<int, int>{0, 0}> ic;
+// { dg-error "static assertion failed" "" { target *-*-* } 0 }