libstdc++: Avoid using sizeof with function types (PR 93470)

	PR libstdc++/93470
	* include/bits/refwrap.h (reference_wrapper::operator()): Restrict
	static assertion to object types.

libstdc++-v3/ChangeLog

@@ -10,6 +10,10 @@
 
 2020-01-28  Jonathan Wakely  <jwakely@redhat.com>
 
+	PR libstdc++/93470
+	* include/bits/refwrap.h (reference_wrapper::operator()): Restrict
+	static assertion to object types.
+
 	PR libstdc++/93325
 	* acinclude.m4 (GLIBCXX_ENABLE_LIBSTDCXX_TIME): Use AC_SEARCH_LIBS for
 	clock_gettime instead of explicit glibc version check.

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

@@ -343,7 +343,8 @@ _GLIBCXX_MEM_FN_TRAITS(&& noexcept, false_type, true_type)
 	operator()(_Args&&... __args) const
 	{
 #if __cplusplus > 201703L
-	  static_assert(sizeof(type), "type must be complete");
+	  if constexpr (is_object_v<type>)
+	    static_assert(sizeof(type), "type must be complete");
 #endif
 	  return std::__invoke(get(), std::forward<_Args>(__args)...);
 	}