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H-G-HristovZingamphilnik777
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[libc++][string] P3044R2: sub-string_view from string (#147095)
Implements [P3044R2](https://wg21.link/P3044R2) Note: `substr.pass.cpp` is refactored to accommodate the test of `basic_string_view`'s `subview` which is an alias of `substr` without changing the test cases. Closes #148140 # References - cplusplus/draft#7975 - https://wg21.link/string.substr - https://wg21.link/string.view.ops --------- Co-authored-by: Hristo Hristov <[email protected]> Co-authored-by: Nikolas Klauser <[email protected]>
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‎libcxx/docs/FeatureTestMacroTable.rst‎

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@@ -506,6 +506,8 @@ Status
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---------------------------------------------------------- -----------------
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``__cpp_lib_sstream_from_string_view`` ``202306L``
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---------------------------------------------------------- -----------------
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``__cpp_lib_string_subview`` ``202506L``
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---------------------------------------------------------- -----------------
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``__cpp_lib_string_view`` ``202403L``
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---------------------------------------------------------- -----------------
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``__cpp_lib_submdspan`` *unimplemented*

‎libcxx/docs/ReleaseNotes/22.rst‎

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@@ -40,6 +40,7 @@ Implemented Papers
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- P2321R2: ``zip`` (`Github <https://llvm.org/PR105169>`__) (The paper is partially implemented. ``zip_transform_view``
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is implemented in this release)
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- P3044R2: sub-``string_view`` from ``string`` (`Github <https://llvm.org/PR148140>`__)
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- P3168R2: Give ``std::optional`` Range Support (`Github <https://llvm.org/PR105430>`__)
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Improvements and New Features

‎libcxx/docs/Status/Cxx2cPapers.csv‎

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@@ -129,7 +129,7 @@
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"`P3179R9 <https://wg21.link/P3179R9>`__","Parallel Range Algorithms","2025-06 (Sofia)","","","`#148137 <https://github.com/llvm/llvm-project/issues/148137>`__",""
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"`P3709R2 <https://wg21.link/P3709R2>`__","Reconsider parallel ``ranges::rotate_copy`` and ``ranges::reverse_copy``","2025-06 (Sofia)","","","`#148138 <https://github.com/llvm/llvm-project/issues/148138>`__",""
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"`P3641R0 <https://wg21.link/P3641R0>`__","Rename ``std::observable`` to ``std::observable_checkpoint``, and add a feature-test macro","2025-06 (Sofia)","","","`#148139 <https://github.com/llvm/llvm-project/issues/148139>`__",""
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"`P3044R2 <https://wg21.link/P3044R2>`__","sub-``string_view`` from ``string``","2025-06 (Sofia)","","","`#148140 <https://github.com/llvm/llvm-project/issues/148140>`__",""
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"`P3044R2 <https://wg21.link/P3044R2>`__","sub-``string_view`` from ``string``","2025-06 (Sofia)","|Complete|","22","`#148140 <https://github.com/llvm/llvm-project/issues/148140>`__",""
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"`P2876R3 <https://wg21.link/P2876R3>`__","Proposal to extend ``std::simd`` with more constructors and accessors","2025-06 (Sofia)","","","`#148143 <https://github.com/llvm/llvm-project/issues/148143>`__",""
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"`P3480R6 <https://wg21.link/P3480R6>`__","``std::simd`` is a range","2025-06 (Sofia)","","","`#148144 <https://github.com/llvm/llvm-project/issues/148144>`__",""
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"`P2664R11 <https://wg21.link/P2664R11>`__","Extend ``std::simd`` with permutation API","2025-06 (Sofia)","","","`#148145 <https://github.com/llvm/llvm-project/issues/148145>`__",""

‎libcxx/include/string‎

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@@ -280,6 +280,8 @@ public:
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basic_string substr(size_type pos = 0, size_type n = npos) const; // constexpr in C++20, removed in C++23
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basic_string substr(size_type pos = 0, size_type n = npos) const&; // since C++23
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constexpr basic_string substr(size_type pos = 0, size_type n = npos) &&; // since C++23
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constexpr basic_string_view<charT, traits> subview(size_type pos = 0,
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size_type n = npos) const; // since C++26
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void swap(basic_string& str)
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noexcept(allocator_traits<allocator_type>::propagate_on_container_swap::value ||
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allocator_traits<allocator_type>::is_always_equal::value); // C++17, constexpr since C++20
@@ -1758,6 +1760,11 @@ public:
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return basic_string(std::move(*this), __pos, __n);
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}
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# endif
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# if _LIBCPP_STD_VER >= 26
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[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr __self_view subview(size_type __pos = 0, size_type __n = npos) const {
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return __self_view(*this).subview(__pos, __n);
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}
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# endif
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_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void swap(basic_string& __str)
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# if _LIBCPP_STD_VER >= 14

‎libcxx/include/string_view‎

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@@ -130,6 +130,8 @@ namespace std {
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size_type copy(charT* s, size_type n, size_type pos = 0) const; // constexpr in C++20
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constexpr basic_string_view substr(size_type pos = 0, size_type n = npos) const;
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constexpr basic_string_view subview(size_type pos = 0,
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size_type n = npos) const; // freestanding-deleted, since C++26
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constexpr int compare(basic_string_view s) const noexcept;
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constexpr int compare(size_type pos1, size_type n1, basic_string_view s) const;
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constexpr int compare(size_type pos1, size_type n1,
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: basic_string_view(__assume_valid(), data() + __pos, std::min(__n, size() - __pos));
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}
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# if _LIBCPP_STD_VER >= 26
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[[nodiscard]]
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_LIBCPP_HIDE_FROM_ABI constexpr basic_string_view subview(size_type __pos = 0, size_type __n = npos) const {
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return substr(__pos, __n);
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}
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# endif
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_LIBCPP_CONSTEXPR_SINCE_CXX14 int compare(basic_string_view __sv) const _NOEXCEPT {
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size_type __rlen = std::min(size(), __sv.size());
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int __retval = _Traits::compare(data(), __sv.data(), __rlen);

‎libcxx/include/version‎

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@@ -245,6 +245,7 @@ __cpp_lib_starts_ends_with 201711L <string> <string
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__cpp_lib_stdatomic_h 202011L <stdatomic.h>
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__cpp_lib_string_contains 202011L <string> <string_view>
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__cpp_lib_string_resize_and_overwrite 202110L <string>
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__cpp_lib_string_subview 202506L <string> <string_view>
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__cpp_lib_string_udls 201304L <string>
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__cpp_lib_string_view 202403L <string> <string_view>
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201803L // C++20
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# define __cpp_lib_span_at 202311L
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# define __cpp_lib_span_initializer_list 202311L
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# define __cpp_lib_sstream_from_string_view 202306L
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# define __cpp_lib_string_subview 202506L
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# undef __cpp_lib_string_view
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# define __cpp_lib_string_view 202403L
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// # define __cpp_lib_submdspan 202306L

‎libcxx/test/libcxx/diagnostics/string.nodiscard.verify.cpp‎

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#include <string>
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#include "test_macros.h"
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void test() {
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std::string string;
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string.empty(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
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#if TEST_STD_VER >= 26
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string.subview(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
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#endif
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}

‎libcxx/test/libcxx/diagnostics/string_view.nodiscard.verify.cpp‎

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#include <string_view>
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#include "test_macros.h"
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void test() {
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std::string_view string_view;
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string_view.empty(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
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#if TEST_STD_VER >= 26
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string_view.subview(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
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#endif
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}

‎libcxx/test/std/language.support/support.limits/support.limits.general/string.version.compile.pass.cpp‎

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# error "__cpp_lib_string_resize_and_overwrite should not be defined before c++23"
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# endif
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# ifdef __cpp_lib_string_subview
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# error "__cpp_lib_string_subview should not be defined before c++26"
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# endif
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# ifdef __cpp_lib_string_udls
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# error "__cpp_lib_string_udls should not be defined before c++14"
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# endif
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# error "__cpp_lib_string_resize_and_overwrite should not be defined before c++23"
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# endif
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# ifdef __cpp_lib_string_subview
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# error "__cpp_lib_string_subview should not be defined before c++26"
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# endif
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# ifndef __cpp_lib_string_udls
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# error "__cpp_lib_string_udls should be defined in c++14"
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# endif
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# error "__cpp_lib_string_resize_and_overwrite should not be defined before c++23"
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# endif
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# ifdef __cpp_lib_string_subview
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# error "__cpp_lib_string_subview should not be defined before c++26"
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# endif
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# ifndef __cpp_lib_string_udls
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# error "__cpp_lib_string_udls should be defined in c++17"
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# endif
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# error "__cpp_lib_string_resize_and_overwrite should not be defined before c++23"
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# endif
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# ifdef __cpp_lib_string_subview
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# error "__cpp_lib_string_subview should not be defined before c++26"
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# endif
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# ifndef __cpp_lib_string_udls
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# error "__cpp_lib_string_udls should be defined in c++20"
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# endif
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# error "__cpp_lib_string_resize_and_overwrite should have the value 202110L in c++23"
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# endif
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# ifdef __cpp_lib_string_subview
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# error "__cpp_lib_string_subview should not be defined before c++26"
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# endif
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# ifndef __cpp_lib_string_udls
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# error "__cpp_lib_string_udls should be defined in c++23"
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# endif
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# error "__cpp_lib_string_resize_and_overwrite should have the value 202110L in c++26"
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# endif
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# ifndef __cpp_lib_string_subview
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# error "__cpp_lib_string_subview should be defined in c++26"
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# endif
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# if __cpp_lib_string_subview != 202506L
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# error "__cpp_lib_string_subview should have the value 202506L in c++26"
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# endif
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# ifndef __cpp_lib_string_udls
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# error "__cpp_lib_string_udls should be defined in c++26"
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# endif

‎libcxx/test/std/language.support/support.limits/support.limits.general/string_view.version.compile.pass.cpp‎

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# error "__cpp_lib_string_contains should not be defined before c++23"
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# endif
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# ifdef __cpp_lib_string_subview
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# error "__cpp_lib_string_subview should not be defined before c++26"
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# endif
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# ifdef __cpp_lib_string_view
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# error "__cpp_lib_string_view should not be defined before c++17"
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# endif
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# error "__cpp_lib_string_contains should not be defined before c++23"
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# endif
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# ifdef __cpp_lib_string_subview
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# error "__cpp_lib_string_subview should not be defined before c++26"
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# endif
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# ifdef __cpp_lib_string_view
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# error "__cpp_lib_string_view should not be defined before c++17"
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# endif
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# error "__cpp_lib_string_contains should not be defined before c++23"
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# endif
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# ifdef __cpp_lib_string_subview
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# error "__cpp_lib_string_subview should not be defined before c++26"
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# endif
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# ifndef __cpp_lib_string_view
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# error "__cpp_lib_string_view should be defined in c++17"
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# endif
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# error "__cpp_lib_string_contains should not be defined before c++23"
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# endif
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# ifdef __cpp_lib_string_subview
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# error "__cpp_lib_string_subview should not be defined before c++26"
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# endif
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# ifndef __cpp_lib_string_view
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# error "__cpp_lib_string_view should be defined in c++20"
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# endif
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# error "__cpp_lib_string_contains should have the value 202011L in c++23"
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# endif
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# ifdef __cpp_lib_string_subview
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# error "__cpp_lib_string_subview should not be defined before c++26"
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# endif
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# ifndef __cpp_lib_string_view
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# error "__cpp_lib_string_view should be defined in c++23"
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# endif
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# error "__cpp_lib_string_contains should have the value 202011L in c++26"
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# endif
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# ifndef __cpp_lib_string_subview
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# error "__cpp_lib_string_subview should be defined in c++26"
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# endif
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# if __cpp_lib_string_subview != 202506L
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# error "__cpp_lib_string_subview should have the value 202506L in c++26"
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# endif
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# ifndef __cpp_lib_string_view
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# error "__cpp_lib_string_view should be defined in c++26"
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# endif

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