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

Backport from mainline
2020-01-28  Jonathan Wakely  <jwakely@redhat.com>

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

libstdc++-v3/ChangeLog

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index bcac851..e997841 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,19 @@
+2020-03-05  Jonathan Wakely  <jwakely@redhat.com>
+
+	Backport from mainline
+	2020-01-28  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/93470
+	* include/bits/refwrap.h (reference_wrapper::operator()): Restrict
+	static assertion to object types.
+
+2020-01-28  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/93325
+	* acinclude.m4 (GLIBCXX_ENABLE_LIBSTDCXX_TIME): Use AC_SEARCH_LIBS for
+	clock_gettime instead of explicit glibc version check.
+	* configure: Regenerate.
+
 2020-03-01  H.J. Lu  <hongjiu.lu@intel.com>
 
 	* config/abi/post/x86_64-linux-gnu/x32/baseline_symbols.txt: Updated.

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

diff --git a/libstdc++-v3/include/bits/refwrap.h b/libstdc++-v3/include/bits/refwrap.h
index 6b4335a..d95f7c5 100644
--- a/libstdc++-v3/include/bits/refwrap.h
+++ b/libstdc++-v3/include/bits/refwrap.h
@@ -334,7 +334,8 @@
 	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)...);
 	}