[PATCH] PR libstdc++/89194 untangle is_convertible and is_nothrow_convertible

[Jonathan Wakely]

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.

Tested powerpc64le-linux, committed to trunk.



commit 0fd1d35fad359e5406deb936688e228f96447ee9
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Tue Feb 5 15:16:09 2019 +0000

    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.

diff --git a/libstdc++-v3/include/std/type_traits 
b/libstdc++-v3/include/std/type_traits
index 7c7aeeb3b32..f05a583cb04 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/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