[gcc r12-4383] libstdc++: Fix regression in memory use when constructing paths

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:f874a13ca3870a56036a90758b0d41c8c217f4f7

commit r12-4383-gf874a13ca3870a56036a90758b0d41c8c217f4f7
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Oct 13 21:32:14 2021 +0100

    libstdc++: Fix regression in memory use when constructing paths
    
    The changes in r12-4381 were intended to reduce memory usage, but
    replacing the __contiguous constant in __string_from_range with the new
    __is_contiguous variable template caused a regression. The old code
    checked is_pointer_v<decltype(std::__niter_base(__first))> but he new
    code just checks is_pointer_v<_InputIterator>. This means that we no
    longer recognise basic_string::iterator and vector::iterator as
    contiguous, and so return a temporary basic_string instead of a
    basic_string_view. This only affects C++17 mode, because the
    std::contiguous_iterator concept is used in C++20 which gives the right
    answer for __normal_iterator (and more types as well).
    
    The fix is to specialize the new __is_contiguous variable template so it
    is true for __normal_iterator<T*, C> specializations. The new partial
    specializations are defined for C++20 too, because it should be cheaper
    to match the partial specialization than to check whether the
    std::contiguous_iterator concept is satisfied.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/fs_path.h (__detail::__is_contiguous): Add
            partial specializations for pointers and __normal_iterator.

Diff:
---
 libstdc++-v3/include/bits/fs_path.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/fs_path.h b/libstdc++-v3/include/bits/fs_path.h
index 05db792fbae..c51bfa3095a 100644
--- a/libstdc++-v3/include/bits/fs_path.h
+++ b/libstdc++-v3/include/bits/fs_path.h
@@ -158,9 +158,16 @@ namespace __detail
     constexpr bool __is_contiguous = std::contiguous_iterator<_Iter>;
 #else
   template<typename _Iter>
-    constexpr bool __is_contiguous = is_pointer_v<_Iter>;
+    constexpr bool __is_contiguous = false;
 #endif
 
+  template<typename _Tp>
+    constexpr bool __is_contiguous<_Tp*> = true;
+
+  template<typename _Tp, typename _Seq>
+    constexpr bool
+    __is_contiguous<__gnu_cxx::__normal_iterator<_Tp*, _Seq>> = true;
+
 #if !defined _GLIBCXX_FILESYSTEM_IS_WINDOWS && defined _GLIBCXX_USE_CHAR8_T
   // For POSIX treat char8_t sequences as char without encoding conversions.
   template<typename _EcharT>