libstdc++: Add const to hash<coroutine_handle<P>>::operator() [PR109165]

libstdc++-v3/ChangeLog:

	PR libstdc++/109165
	* include/std/coroutine (hash<>::operator()): Add const.
	* testsuite/18_support/coroutines/hash.cc: New test.

libstdc++-v3/include/std/coroutine

@@ -345,7 +345,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     struct hash<coroutine_handle<_Promise>>
     {
       size_t
-      operator()(const coroutine_handle<_Promise>& __h) noexcept
+      operator()(const coroutine_handle<_Promise>& __h) const noexcept
       {
 	return reinterpret_cast<size_t>(__h.address());
       }

libstdc++-v3/testsuite/18_support/coroutines/hash.cc

+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <coroutine>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  std::hash<std::noop_coroutine_handle> h;
+  std::size_t v = h(std::noop_coroutine());
+
+  const auto& ch = h;
+  std::size_t v2 = h(std::noop_coroutine()); // PR libstdc++/109165
+
+  VERIFY( v2 == v );
+}
+
+int main()
+{
+  test01();
+}