PR libstdc++/82254 fix std::is_nothrow_invocable_r w.r.t throwing conversions

	PR libstdc++/82254
	* include/std/type_traits (__is_invocable): Add partial specialization
	for INVOKE<void> case and remove is_void<R> check from partial
	specialization for INVOKE<R> case.
	(__is_nt_invocable_impl): New helper for is_nothrow_invocable_r.
	(is_nothrow_invocable_r): Use __is_nt_invocable_impl.
	* testsuite/20_util/is_nothrow_invocable/value.cc: Add tests for
	conversions that can throw or fail to convert. Use static assert
	strings to explain negative results.
	* testsuite/20_util/is_nothrow_invocable/value_ext.cc: Use
	is_nothrow_constructible in is_nt_invocable_conv.

From-SVN: r252977

4 files changed, 115 insertions, 24 deletions

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 28a9609..3c75c76 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,17 @@
+2017-09-19  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/82254
+	* include/std/type_traits (__is_invocable): Add partial specialization
+	for INVOKE<void> case and remove is_void<R> check from partial
+	specialization for INVOKE<R> case.
+	(__is_nt_invocable_impl): New helper for is_nothrow_invocable_r.
+	(is_nothrow_invocable_r): Use __is_nt_invocable_impl.
+	* testsuite/20_util/is_nothrow_invocable/value.cc: Add tests for
+	conversions that can throw or fail to convert. Use static assert
+	strings to explain negative results.
+	* testsuite/20_util/is_nothrow_invocable/value_ext.cc: Use
+	is_nothrow_constructible in is_nt_invocable_conv.
+
 2017-09-18  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/81468
diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 15b0d92..036f766 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -2592,7 +2592,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   template<typename _Result, typename _Ret>
     struct __is_invocable_impl<_Result, _Ret, __void_t<typename _Result::type>>
-    : __or_<is_void<_Ret>, is_convertible<typename _Result::type, _Ret>>::type
+    : is_convertible<typename _Result::type, _Ret>::type
+    { };
+
+  template<typename _Result>
+    struct __is_invocable_impl<_Result, void, __void_t<typename _Result::type>>
+    : true_type
     { };
 
   template<typename _Fn, typename... _ArgTypes>
@@ -2691,10 +2696,26 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
              __call_is_nothrow_<_Fn, _ArgTypes...>>::type
     { };
 
+  template<typename _Result, typename _Ret, typename = void>
+    struct __is_nt_invocable_impl : false_type { };
+
+  template<typename _Result, typename _Ret>
+    struct __is_nt_invocable_impl<_Result, _Ret,
+				  __void_t<typename _Result::type>>
+    : __and_<is_convertible<typename _Result::type, _Ret>,
+	     is_nothrow_constructible<_Ret, typename _Result::type>>
+    { };
+
+  template<typename _Result>
+    struct __is_nt_invocable_impl<_Result, void,
+				  __void_t<typename _Result::type>>
+    : true_type
+    { };
+
   /// std::is_nothrow_invocable_r
   template<typename _Ret, typename _Fn, typename... _ArgTypes>
     struct is_nothrow_invocable_r
-    : __and_<__is_invocable_impl<__invoke_result<_Fn, _ArgTypes...>, _Ret>,
+    : __and_<__is_nt_invocable_impl<__invoke_result<_Fn, _ArgTypes...>, _Ret>,
              __call_is_nothrow_<_Fn, _ArgTypes...>>::type
     { };
 
diff --git a/libstdc++-v3/testsuite/20_util/is_nothrow_invocable/value.cc b/libstdc++-v3/testsuite/20_util/is_nothrow_invocable/value.cc
index 4ccb459..dfa76aa 100644
--- a/libstdc++-v3/testsuite/20_util/is_nothrow_invocable/value.cc
+++ b/libstdc++-v3/testsuite/20_util/is_nothrow_invocable/value.cc
@@ -40,6 +40,10 @@ template<typename R, typename... T>
 
 void test01()
 {
+  struct T { T(int) { } };
+  struct NT { NT(int) noexcept { } };
+  struct Ex { explicit Ex(int) noexcept { } };
+
   using func_type = void(*)();
   static_assert( ! is_nt_invocable< func_type>(),     "");
 
@@ -55,28 +59,46 @@ void test01()
   static_assert( ! is_nt_invocable< mem_type, int >(),   "");
   static_assert( ! is_nt_invocable< mem_type, int& >(),	"");
 
-  static_assert(   is_nt_invocable< mem_type, X& >(),       "");
-  static_assert(   is_nt_invocable_r< int,  mem_type, X& >(), "");
-  static_assert(   is_nt_invocable_r< int&, mem_type, X& >(), "");
-  static_assert(   is_nt_invocable_r< long, mem_type, X& >(), "");
-  static_assert(   is_nt_invocable_r< int&, mem_type, X* >(), "");
+  static_assert(   is_nt_invocable< mem_type, X& >(),          "");
+  static_assert(   is_nt_invocable_r< int,   mem_type, X& >(), "");
+  static_assert(   is_nt_invocable_r< int&,  mem_type, X& >(), "");
+  static_assert(   is_nt_invocable_r< long,  mem_type, X& >(), "");
+  static_assert( ! is_nt_invocable_r< long&, mem_type, X& >(),
+		   "conversion fails, cannot bind long& to int");
+  static_assert(   is_nt_invocable_r< int&,  mem_type, X* >(), "");
+
+  static_assert( ! is_nt_invocable_r< T,  mem_type, X& >(),
+		   "conversion throws");
+  static_assert(   is_nt_invocable_r< NT, mem_type, X& >(), "");
+  static_assert( ! is_nt_invocable_r< Ex, mem_type, X& >(),
+		   "conversion fails, would use explicit constructor");
 
   using memfun_type = int (X::*)();
 
-  static_assert( ! is_nt_invocable< memfun_type >(),     "");
-  static_assert( ! is_nt_invocable< memfun_type, int >(),  "");
-  static_assert( ! is_nt_invocable< memfun_type, int& >(), "");
-  static_assert( ! is_nt_invocable< memfun_type, X& >(),   "");
-  static_assert( ! is_nt_invocable< memfun_type, X* >(),   "");
+  static_assert( ! is_nt_invocable< memfun_type >(),       "no object");
+  static_assert( ! is_nt_invocable< memfun_type, int >(),  "no object");
+  static_assert( ! is_nt_invocable< memfun_type, int& >(), "no object");
+  static_assert( ! is_nt_invocable< memfun_type, X& >(),   "call throws");
+  static_assert( ! is_nt_invocable< memfun_type, X* >(),   "call throws");
+
+  static_assert( ! is_nt_invocable_r< T,  memfun_type, X& >(), "call throws");
+  static_assert( ! is_nt_invocable_r< NT, memfun_type, X& >(), "call throws");
+  static_assert( ! is_nt_invocable_r< Ex, memfun_type, X& >(), "call throws");
 
 #if __cpp_noexcept_function_type
   using memfun_type_nt = int (X::*)() noexcept;
 
-  static_assert( ! is_nt_invocable< memfun_type_nt >(),	    "");
-  static_assert( ! is_nt_invocable< memfun_type_nt, int >(),  "");
-  static_assert( ! is_nt_invocable< memfun_type_nt, int& >(), "");
+  static_assert( ! is_nt_invocable< memfun_type_nt >(),	      "no object");
+  static_assert( ! is_nt_invocable< memfun_type_nt, int >(),  "no object");
+  static_assert( ! is_nt_invocable< memfun_type_nt, int& >(), "no object");
   static_assert(   is_nt_invocable< memfun_type_nt, X& >(),   "");
   static_assert(   is_nt_invocable< memfun_type_nt, X* >(),   "");
+
+  static_assert( ! is_nt_invocable_r< T,  memfun_type_nt, X& >(),
+		   "conversion throws");
+  static_assert(   is_nt_invocable_r< NT, memfun_type_nt, X& >(), "");
+  static_assert( ! is_nt_invocable_r< Ex, memfun_type_nt, X& >(),
+		   "conversion fails, would use explicit constructor");
 #endif
 
   struct F {
@@ -89,12 +111,44 @@ void test01()
   };
   using CF = const F;
 
-  static_assert( ! is_nt_invocable_r< int&,  F  >(), "");
-  static_assert(   is_nt_invocable_r< long&, CF >(),  "");
-  static_assert( ! is_nt_invocable_r< short&, F,   int >(), "" );
-  static_assert(   is_nt_invocable_r< char&,  F&,  int >(), "" );
-  static_assert(   is_nt_invocable_r< char&,  CF,  int >(), "" );
-  static_assert(   is_nt_invocable_r< char&,  CF&, int >(), "" );
-
-  static_assert( ! is_nt_invocable< F, int, int >(), "");
+  static_assert( ! is_nt_invocable< F  >(), "call throws");
+  static_assert(   is_nt_invocable< CF >(), "");
+
+  static_assert( ! is_nt_invocable_r< int&,  F  >(), "call throws");
+  static_assert(   is_nt_invocable_r< long&, CF >(), "");
+  static_assert( ! is_nt_invocable_r< T,     F  >(), "call throws");
+  static_assert( ! is_nt_invocable_r< NT,    F  >(), "call throws");
+  static_assert( ! is_nt_invocable_r< Ex,    F  >(), "call throws");
+  static_assert( ! is_nt_invocable_r< T,     CF >(), "conversion throws");
+  static_assert(   is_nt_invocable_r< NT,    CF >(), "" );
+  static_assert( ! is_nt_invocable_r< Ex,    CF >(), "conversion fails");
+
+  static_assert( ! is_nt_invocable< F,   int >(), "call throws");
+  static_assert(   is_nt_invocable< F&,  int >(), "");
+
+  static_assert( ! is_nt_invocable_r< short&, F,   int >(),
+		   "call throws" );
+  static_assert(   is_nt_invocable_r< char&,  F&,  int >(), "");
+  static_assert( ! is_nt_invocable_r< T,      F&,  int >(),
+		   "conversion throws");
+  static_assert(   is_nt_invocable_r< NT,     F&,  int >(), "");
+  static_assert( ! is_nt_invocable_r< Ex,     F&,  int >(),
+		   "conversion fails, would use explicit constructor");
+
+  static_assert(   is_nt_invocable< CF,   int >(), "");
+  static_assert(   is_nt_invocable< CF&,  int >(), "");
+
+  static_assert(   is_nt_invocable_r< char&,  CF,  int >(), "");
+  static_assert(   is_nt_invocable_r< char&,  CF&, int >(), "");
+
+  static_assert( ! is_nt_invocable_r< T,      CF&, int >(),
+		   "conversion throws");
+  static_assert(   is_nt_invocable_r< NT,     CF&, int >(), "");
+  static_assert( ! is_nt_invocable_r< Ex,     CF&, int >(),
+		   "conversion fails, would use explicit constructor");
+
+  static_assert( ! is_nt_invocable< F, int, int >(),
+		   "would call private member");
+  static_assert( ! is_nt_invocable_r<void, F, int, int >(),
+		   "would call private member");
 }
diff --git a/libstdc++-v3/testsuite/20_util/is_nothrow_invocable/value_ext.cc b/libstdc++-v3/testsuite/20_util/is_nothrow_invocable/value_ext.cc
index 7217324..7fd3d92 100644
--- a/libstdc++-v3/testsuite/20_util/is_nothrow_invocable/value_ext.cc
+++ b/libstdc++-v3/testsuite/20_util/is_nothrow_invocable/value_ext.cc
@@ -27,7 +27,9 @@ template<typename... T>
   constexpr bool is_nt_invocable_conv(std::true_type)
   {
     using result_type = typename std::__invoke_result<T...>::type;
-    return std::is_void<R>::value || std::is_convertible<result_type, R>::value;
+    return std::is_void<R>::value
+      || (std::is_convertible<result_type, R>::value
+	  && std::is_nothrow_constructible<R, result_type>::value);
   }
 
 template<typename R, typename... T>