[PATCH] Simplify common case of use_future_t that uses std::allocator

[Jonathan Wakely <jwakely@redhat.com>]

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There is no need to store and pass around the allocator object when it's
an instance of std::allocator. Define a partial specialization of
std::use_future_t and the corresponding completion token so that no
allocator is stored. Overload the completion handler constructor to not
expect an allocator to be stored.

	* include/experimental/executor (__use_future_ct, use_future_t):
	Define partial specializations for std::allocator.
	(__use_future_ch): Overload constructor for completion tokens using
	std::allocator.

Tested x86_64-linux, committed to trunk.



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commit ff1859de72d39f46118df4f568ee90d78c6be937
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Oct 24 15:34:17 2019 +0100

    Simplify common case of use_future_t that uses std::allocator
    
    There is no need to store and pass around the allocator object when it's
    an instance of std::allocator. Define a partial specialization of
    std::use_future_t and the corresponding completion token so that no
    allocator is stored. Overload the completion handler constructor to not
    expect an allocator to be stored.
    
            * include/experimental/executor (__use_future_ct, use_future_t):
            Define partial specializations for std::allocator.
            (__use_future_ch): Overload constructor for completion tokens using
            std::allocator.

diff --git a/libstdc++-v3/include/experimental/executor b/libstdc++-v3/include/experimental/executor
index ef141e92cd3..ed18730951c 100644
--- a/libstdc++-v3/include/experimental/executor
+++ b/libstdc++-v3/include/experimental/executor
@@ -1501,12 +1501,18 @@ inline namespace v1
       std::tuple<_Func, _Alloc> _M_t;
     };
 
+  template<typename _Func, typename _Tp>
+    struct __use_future_ct<_Func, std::allocator<_Tp>>
+    {
+      _Func _M_f;
+    };
+
   template<typename _ProtoAllocator = allocator<void>>
     class use_future_t
     {
     public:
       // use_future_t types:
-      typedef _ProtoAllocator allocator_type;
+      using allocator_type = _ProtoAllocator;
 
       // use_future_t members:
       constexpr use_future_t() noexcept : _M_alloc() { }
@@ -1514,7 +1520,7 @@ inline namespace v1
       explicit
       use_future_t(const _ProtoAllocator& __a) noexcept : _M_alloc(__a) { }
 
-      template<class _OtherAllocator>
+      template<typename _OtherAllocator>
 	use_future_t<_OtherAllocator>
 	rebind(const _OtherAllocator& __a) const noexcept
 	{ return use_future_t<_OtherAllocator>(__a); }
@@ -1533,6 +1539,35 @@ inline namespace v1
       _ProtoAllocator _M_alloc;
     };
 
+  template<typename _Tp>
+    class use_future_t<std::allocator<_Tp>>
+    {
+    public:
+      // use_future_t types:
+      using allocator_type = std::allocator<_Tp>;
+
+      // use_future_t members:
+      constexpr use_future_t() noexcept = default;
+
+      explicit
+      use_future_t(const allocator_type& __a) noexcept { }
+
+      template<class _Up>
+	use_future_t<std::allocator<_Up>>
+	rebind(const std::allocator<_Up>& __a) const noexcept
+	{ return use_future_t<std::allocator<_Up>>(__a); }
+
+      allocator_type get_allocator() const noexcept { return {}; }
+
+      template<typename _Func>
+	auto
+	operator()(_Func&& __f) const
+	{
+	  using _Token = __use_future_ct<decay_t<_Func>, allocator_type>;
+	  return _Token{std::forward<_Func>(__f)};
+	}
+    };
+
   constexpr use_future_t<> use_future = use_future_t<>();
 
   template<typename _Func, typename _Alloc, typename _Res, typename... _Args>
@@ -1552,6 +1587,12 @@ inline namespace v1
 	  _M_promise{ std::get<1>(__token._M_t) }
 	{ }
 
+      template<typename _Tp>
+	explicit
+	__use_future_ch(__use_future_ct<_Func, std::allocator<_Tp>>&& __token)
+	: _M_f{ std::move(__token._M_f) }
+	{ }
+
       void
       operator()(_Args&&... __args)
       {