[gcc r12-1486] libstdc++: Use function object for __decay_copy helper

[Jonathan Wakely]

https://gcc.gnu.org/g:cb326a6442f09cb36b05ce556fc91e10bfeb0cf6

commit r12-1486-gcb326a6442f09cb36b05ce556fc91e10bfeb0cf6
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Jun 15 14:39:02 2021 +0100

    libstdc++: Use function object for __decay_copy helper
    
    By changing __cust_access::__decay_copy from a function template to a
    function object we avoid ADL. That means it's fine to call it
    unqualified (the compiler won't waste time doing ADL in associated
    namespaces, and won't try to complete associated types).
    
    This also makes some other minor simplications to other concepts for the
    [range.access] CPOs.
    
    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/iterator_concepts.h (__cust_access::__decay_copy):
            Replace with function object.
            (__cust_access::__member_begin, ___cust_access::_adl_begin): Use
            __decay_copy unqualified.
            * include/bits/ranges_base.h (__member_end, __adl_end):
            Likewise. Use __range_iter_t for type of ranges::begin.
            (__member_rend): Use correct value category for rbegin argument.
            (__member_data): Use __decay_copy unqualified.
            (__begin_data): Use __range_iter_t for type of ranges::begin.

Diff:
---
 libstdc++-v3/include/bits/iterator_concepts.h | 18 +++++++++---------
 libstdc++-v3/include/bits/ranges_base.h       | 14 ++++++--------
 2 files changed, 15 insertions(+), 17 deletions(-)

diff --git a/libstdc++-v3/include/bits/iterator_concepts.h b/libstdc++-v3/include/bits/iterator_concepts.h
index 8723f35c5cb..d18ae32bf20 100644
--- a/libstdc++-v3/include/bits/iterator_concepts.h
+++ b/libstdc++-v3/include/bits/iterator_concepts.h
@@ -930,17 +930,18 @@ namespace ranges
   {
     using std::__detail::__class_or_enum;
 
-    template<typename _Tp>
-      constexpr decay_t<_Tp>
-      __decay_copy(_Tp&& __t)
-      noexcept(is_nothrow_convertible_v<_Tp, decay_t<_Tp>>)
-      { return std::forward<_Tp>(__t); }
+    struct {
+      template<typename _Tp>
+	constexpr decay_t<_Tp>
+	operator()(_Tp&& __t) const
+	noexcept(is_nothrow_convertible_v<_Tp, decay_t<_Tp>>)
+	{ return std::forward<_Tp>(__t); }
+    } inline constexpr __decay_copy{};
 
     template<typename _Tp>
       concept __member_begin = requires(_Tp& __t)
 	{
-	  { __cust_access::__decay_copy(__t.begin()) }
-	    -> input_or_output_iterator;
+	  { __decay_copy(__t.begin()) } -> input_or_output_iterator;
 	};
 
     // Poison pills so that unqualified lookup doesn't find std::begin.
@@ -951,8 +952,7 @@ namespace ranges
       concept __adl_begin = __class_or_enum<remove_reference_t<_Tp>>
 	&& requires(_Tp& __t)
 	{
-	  { __cust_access::__decay_copy(begin(__t)) }
-	    -> input_or_output_iterator;
+	  { __decay_copy(begin(__t)) } -> input_or_output_iterator;
 	};
 
     // Simplified version of std::ranges::begin that only supports lvalues,
diff --git a/libstdc++-v3/include/bits/ranges_base.h b/libstdc++-v3/include/bits/ranges_base.h
index 3bc657ca17e..a63ef8eb7f4 100644
--- a/libstdc++-v3/include/bits/ranges_base.h
+++ b/libstdc++-v3/include/bits/ranges_base.h
@@ -89,6 +89,7 @@ namespace ranges
   namespace __cust_access
   {
     using std::ranges::__detail::__maybe_borrowed_range;
+    using std::__detail::__range_iter_t;
 
     struct _Begin
     {
@@ -127,8 +128,7 @@ namespace ranges
     template<typename _Tp>
       concept __member_end = requires(_Tp& __t)
 	{
-	  { __decay_copy(__t.end()) }
-	    -> sentinel_for<decltype(_Begin{}(std::forward<_Tp>(__t)))>;
+	  { __decay_copy(__t.end()) } -> sentinel_for<__range_iter_t<_Tp>>;
 	};
 
     // Poison pills so that unqualified lookup doesn't find std::end.
@@ -139,8 +139,7 @@ namespace ranges
       concept __adl_end = __class_or_enum<remove_reference_t<_Tp>>
 	&& requires(_Tp& __t)
 	{
-	  { __decay_copy(end(__t)) }
-	    -> sentinel_for<decltype(_Begin{}(std::forward<_Tp>(__t)))>;
+	  { __decay_copy(end(__t)) } -> sentinel_for<__range_iter_t<_Tp>>;
 	};
 
     struct _End
@@ -281,7 +280,7 @@ namespace ranges
       concept __member_rend = requires(_Tp& __t)
 	{
 	  { __decay_copy(__t.rend()) }
-	    -> sentinel_for<decltype(_RBegin{}(__t))>;
+	    -> sentinel_for<decltype(_RBegin{}(std::forward<_Tp>(__t)))>;
 	};
 
     void rend(auto&) = delete;
@@ -507,12 +506,11 @@ namespace ranges
     template<typename _Tp>
       concept __member_data = requires(_Tp& __t)
 	{
-	  { __cust_access::__decay_copy(__t.data()) } -> __pointer_to_object;
+	  { __decay_copy(__t.data()) } -> __pointer_to_object;
 	};
 
     template<typename _Tp>
-      concept __begin_data = requires(_Tp& __t)
-	{ { _Begin{}(__t) } -> contiguous_iterator; };
+      concept __begin_data = contiguous_iterator<__range_iter_t<_Tp>>;
 
     struct _Data
     {