[PATCH] LWG 3039 Unnecessary decay in thread and packaged_task

[Jonathan Wakely]

LWG 3039 changed the spec to use remove_cvref_t instead of decay_t,
and this patch makes that change to our std::packaged_task.  I'm not
sure why I created a custom type at namespace scope to constrain
packaged_task. This patch replaces it with an alias for enable_if.

std::thread already had the change, so this just adds a comment noting
the issue number.

	* include/std/future (__constrain_pkgdtask): Replace with ...
	(packaged_task::__not_same): New alias template, using
	__remove_cvref_t instead of decay.
	* include/std/thread (thread::__not_same): Add comment.

Tested powerpc64le-linux, committed to trunk.


-------------- next part --------------
commit df8c05879aa008a993cbd5f0762f9ef8557f2bb7
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Jun 14 23:36:09 2018 +0100

    LWG 3039 Unnecessary decay in thread and packaged_task
    
            * include/std/future (__constrain_pkgdtask): Replace with ...
            (packaged_task::__not_same): New alias template, using
            __remove_cvref_t instead of decay.
            * include/std/thread (thread::__not_same): Add comment.

diff --git a/libstdc++-v3/include/std/future b/libstdc++-v3/include/std/future
index c17a253d26e..937c05ef2b2 100644
--- a/libstdc++-v3/include/std/future
+++ b/libstdc++-v3/include/std/future
@@ -1462,15 +1462,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 						 static_cast<_Alloc&>(_M_impl));
     }
 
-  template<typename _Task, typename _Fn, bool
-	   = is_same<_Task, typename decay<_Fn>::type>::value>
-    struct __constrain_pkgdtask
-    { typedef void __type; };
-
-  template<typename _Task, typename _Fn>
-    struct __constrain_pkgdtask<_Task, _Fn, true>
-    { };
-
   /// packaged_task
   template<typename _Res, typename... _ArgTypes>
     class packaged_task<_Res(_ArgTypes...)>
@@ -1478,6 +1469,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       typedef __future_base::_Task_state_base<_Res(_ArgTypes...)> _State_type;
       shared_ptr<_State_type>                   _M_state;
 
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3039. Unnecessary decay in thread and packaged_task
+      template<typename _Fn, typename _Fn2 = __remove_cvref_t<_Fn>>
+	using __not_same
+	  = typename enable_if<!is_same<packaged_task, _Fn2>::value>::type;
+
     public:
       // Construction and destruction
       packaged_task() noexcept { }
@@ -1488,8 +1485,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	packaged_task(allocator_arg_t, const _Allocator& __a) noexcept
 	{ }
 
-      template<typename _Fn, typename = typename
-	       __constrain_pkgdtask<packaged_task, _Fn>::__type>
+      template<typename _Fn, typename = __not_same<_Fn>>
 	explicit
 	packaged_task(_Fn&& __fn)
 	: packaged_task(allocator_arg, std::allocator<int>(),
@@ -1499,11 +1495,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // _GLIBCXX_RESOLVE_LIB_DEFECTS
       // 2097.  packaged_task constructors should be constrained
       // 2407. [this constructor should not be] explicit
-      template<typename _Fn, typename _Alloc, typename = typename
-	       __constrain_pkgdtask<packaged_task, _Fn>::__type>
+      template<typename _Fn, typename _Alloc, typename = __not_same<_Fn>>
 	packaged_task(allocator_arg_t, const _Alloc& __a, _Fn&& __fn)
 	: _M_state(__create_task_state<_Res(_ArgTypes...)>(
-		    std::forward<_Fn>(__fn), __a))
+		   std::forward<_Fn>(__fn), __a))
 	{ }
 
       ~packaged_task()
diff --git a/libstdc++-v3/include/std/thread b/libstdc++-v3/include/std/thread
index 61861b58520..13acd6a3091 100644
--- a/libstdc++-v3/include/std/thread
+++ b/libstdc++-v3/include/std/thread
@@ -104,6 +104,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
     // _GLIBCXX_RESOLVE_LIB_DEFECTS
     // 2097.  packaged_task constructors should be constrained
+    // 3039. Unnecessary decay in thread and packaged_task
     template<typename _Tp>
       using __not_same = __not_<is_same<__remove_cvref_t<_Tp>, thread>>;