PR libstdc++/89194 untangle is_convertible and is_nothrow_convertible

The additional logic added to __is_convertible_helper in order to
support is_nothrow_convertible makes some uses of is_convertible
ill-formed. This appears to be due to PR c++/87603, but can be avoided
just by defining a separate helper for is_nothrow_convertible. The same
problems are likely to still exist for is_nothrow_convertible, but that
is new and so won't cause regressions for existing users of
is_convertible.

	PR libstdc++/89194
	* include/std/type_traits (__is_convertible_helper)
	(__is_convertible_helper<_From, _To, false>): Revert changes to
	support is_nothrow_convertible.
	(__is_nt_convertible_helper): New helper.
	(is_nothrow_convertible): Use __is_nt_convertible_helper.

From-SVN: r268543

libstdc++-v3/ChangeLog

 2019-02-05  Jonathan Wakely  <jwakely@redhat.com>
 
+	PR libstdc++/89194
+	* include/std/type_traits (__is_convertible_helper)
+	(__is_convertible_helper<_From, _To, false>): Revert changes to
+	support is_nothrow_convertible.
+	(__is_nt_convertible_helper): New helper.
+	(is_nothrow_convertible): Use __is_nt_convertible_helper.
+
 	* testsuite/23_containers/vector/modifiers/push_back/49836.cc: Restore
 	use of CopyConsOnlyType, but also test DelAnyAssign for completeness.
 

libstdc++-v3/include/std/type_traits

@@ -1344,9 +1344,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     struct __is_convertible_helper
     {
       typedef typename is_void<_To>::type type;
-#if __cplusplus > 201703L
-      typedef type __is_nothrow_type;
-#endif
     };
 
   template<typename _From, typename _To>
@@ -1364,23 +1361,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	static false_type
 	__test(...);
 
-#if __cplusplus > 201703L
-      template<typename _From1, typename _To1,
-	       bool _NoEx = noexcept(__test_aux<_To1>(std::declval<_From1>()))>
-	static __bool_constant<_NoEx>
-	__test_nothrow(int);
-
-      template<typename, typename>
-	static false_type
-	__test_nothrow(...);
-#endif
-
     public:
       typedef decltype(__test<_From, _To>(0)) type;
-
-#if __cplusplus > 201703L
-      typedef decltype(__test_nothrow<_From, _To>(0)) __is_nothrow_type;
-#endif
     };
 
 
@@ -1391,10 +1373,35 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { };
 
 #if __cplusplus > 201703L
+    template<typename _From, typename _To,
+           bool = __or_<is_void<_From>, is_function<_To>,
+                        is_array<_To>>::value>
+    struct __is_nt_convertible_helper
+    : is_void<_To>
+    { };
+
+  template<typename _From, typename _To>
+    class __is_nt_convertible_helper<_From, _To, false>
+    {
+      template<typename _To1>
+	static void __test_aux(_To1) noexcept;
+
+      template<typename _From1, typename _To1>
+	static bool_constant<noexcept(__test_aux<_To1>(std::declval<_From1>()))>
+	__test(int);
+
+      template<typename, typename>
+	static false_type
+	__test(...);
+
+    public:
+      using type = decltype(__test<_From, _To>(0));
+    };
+
   /// is_nothrow_convertible
   template<typename _From, typename _To>
     struct is_nothrow_convertible
-    : public __is_convertible_helper<_From, _To>::__is_nothrow_type
+    : public __is_nt_convertible_helper<_From, _To>::type
     { };
 
   /// is_nothrow_convertible_v