[committed] libstdc++: Use requires-clause for __normal_iterator constructor

[Jonathan Wakely]

This is a very minor throughput optimization, to avoid instantiating
std::enable_if and std::is_convertible when concepts are available.

libstdc++-v3/ChangeLog:

        * include/bits/stl_iterator.h (__normal_iterator): Replace
        enable_if constraint with requires-clause.
---

Tested x86_64-linux. Pushed to trunk.

 libstdc++-v3/include/bits/stl_iterator.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/stl_iterator.h 
b/libstdc++-v3/include/bits/stl_iterator.h
index 8c86cb214b2..5ea901bf1f6 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -1030,7 +1030,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       typedef std::iterator_traits<_Iterator>          __traits_type;
 
-#if __cplusplus >= 201103L
+#if __cplusplus >= 201103L && ! defined __glibcxx_concepts
       template<typename _Iter>
        using __convertible_from
          = std::__enable_if_t<std::is_convertible<_Iter, _Iterator>::value>;
@@ -1060,7 +1060,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       // Allow iterator to const_iterator conversion
 #if __cplusplus >= 201103L
+# ifdef __glibcxx_concepts
+      template<typename _Iter> requires std::is_convertible_v<_Iter, _Iterator>
+# else
       template<typename _Iter, typename = __convertible_from<_Iter>>
+# endif
        [[__gnu__::__always_inline__]]
        constexpr
        __normal_iterator(const __normal_iterator<_Iter, _Container>& __i)
-- 
2.47.0