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.
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@ -158,9 +158,16 @@ namespace __detail
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constexpr bool __is_contiguous = std::contiguous_iterator<_Iter>;
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#else
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template<typename _Iter>
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constexpr bool __is_contiguous = is_pointer_v<_Iter>;
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constexpr bool __is_contiguous = false;
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#endif
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template<typename _Tp>
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constexpr bool __is_contiguous<_Tp*> = true;
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template<typename _Tp, typename _Seq>
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constexpr bool
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__is_contiguous<__gnu_cxx::__normal_iterator<_Tp*, _Seq>> = true;
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#if !defined _GLIBCXX_FILESYSTEM_IS_WINDOWS && defined _GLIBCXX_USE_CHAR8_T
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// For POSIX treat char8_t sequences as char without encoding conversions.
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template<typename _EcharT>
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