PR go/83102 relax std::set checks for invocable comparison object

	PR go/83102
	* include/bits/stl_tree.h (_Rb_tree): Relax invocable checks for
	comparison object pre-C++17.

From-SVN: r255052

libstdc++-v3/ChangeLog

+2017-11-22  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR go/83102
+	* include/bits/stl_tree.h (_Rb_tree): Relax invocable checks for
+	comparison object pre-C++17.
+
 2017-11-21  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/48101

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

@@ -449,9 +449,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       typedef __gnu_cxx::__alloc_traits<_Node_allocator> _Alloc_traits;
 
 #if __cplusplus >= 201103L
-      static_assert(__is_invocable<const _Compare&, const _Key&, const _Key&>{},
+      static_assert(__is_invocable<_Compare&, const _Key&, const _Key&>{},
 	  "comparison object must be invocable with two arguments of key type");
-#endif
+# if __cplusplus >= 201703L
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 2542. Missing const requirements for associative containers
+      static_assert(is_invocable_v<const _Compare&, const _Key&, const _Key&>,
+	  "comparison object must be invocable as const");
+# endif // C++17
+#endif // C++11
 
     protected:
       typedef _Rb_tree_node_base* 		_Base_ptr;