[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]>

Author: 3 people

libcxx/docs/FeatureTestMacroTable.rst

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

libcxx/docs/ReleaseNotes/22.rst

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

libcxx/docs/Status/Cxx2cPapers.csv

@@ -129,7 +129,7 @@
 "`P3179R9 <https://wg21.link/P3179R9>`__","Parallel Range Algorithms","2025-06 (Sofia)","","","`#148137 <https://github.com/llvm/llvm-project/issues/148137>`__",""
 "`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>`__",""
 "`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>`__",""
-"`P3044R2 <https://wg21.link/P3044R2>`__","sub-``string_view`` from ``string``","2025-06 (Sofia)","","","`#148140 <https://github.com/llvm/llvm-project/issues/148140>`__",""
+"`P3044R2 <https://wg21.link/P3044R2>`__","sub-``string_view`` from ``string``","2025-06 (Sofia)","|Complete|","22","`#148140 <https://github.com/llvm/llvm-project/issues/148140>`__",""
 "`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>`__",""
 "`P3480R6 <https://wg21.link/P3480R6>`__","``std::simd`` is a range","2025-06 (Sofia)","","","`#148144 <https://github.com/llvm/llvm-project/issues/148144>`__",""
 "`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

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

libcxx/include/string_view

@@ -130,6 +130,8 @@ namespace std {
       size_type copy(charT* s, size_type n, size_type pos = 0) const;  // constexpr in C++20
 
       constexpr basic_string_view substr(size_type pos = 0, size_type n = npos) const;
+      constexpr basic_string_view subview(size_type pos = 0,
+                                          size_type n = npos) const;      // freestanding-deleted, since C++26
       constexpr int compare(basic_string_view s) const noexcept;
       constexpr int compare(size_type pos1, size_type n1, basic_string_view s) const;
       constexpr int compare(size_type pos1, size_type n1,
@@ -465,6 +467,13 @@ public:
                           : basic_string_view(__assume_valid(), data() + __pos, std::min(__n, size() - __pos));
   }
 
+#  if _LIBCPP_STD_VER >= 26
+  [[nodiscard]]
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_string_view subview(size_type __pos = 0, size_type __n = npos) const {
+    return substr(__pos, __n);
+  }
+#  endif
+
   _LIBCPP_CONSTEXPR_SINCE_CXX14 int compare(basic_string_view __sv) const _NOEXCEPT {
     size_type __rlen = std::min(size(), __sv.size());
     int __retval     = _Traits::compare(data(), __sv.data(), __rlen);

libcxx/include/version

@@ -245,6 +245,7 @@ __cpp_lib_starts_ends_with                              201711L <string> <string
 __cpp_lib_stdatomic_h                                   202011L <stdatomic.h>
 __cpp_lib_string_contains                               202011L <string> <string_view>
 __cpp_lib_string_resize_and_overwrite                   202110L <string>
+__cpp_lib_string_subview                                202506L <string> <string_view>
 __cpp_lib_string_udls                                   201304L <string>
 __cpp_lib_string_view                                   202403L <string> <string_view>
                                                         201803L // C++20
@@ -599,6 +600,7 @@ __cpp_lib_void_t                                        201411L <type_traits>
 # define __cpp_lib_span_at                              202311L
 # define __cpp_lib_span_initializer_list                202311L
 # define __cpp_lib_sstream_from_string_view             202306L
+# define __cpp_lib_string_subview                       202506L
 # undef  __cpp_lib_string_view
 # define __cpp_lib_string_view                          202403L
 // # define __cpp_lib_submdspan                            202306L

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

@@ -12,7 +12,12 @@
 
 #include <string>
 
+#include "test_macros.h"
+
 void test() {
   std::string string;
   string.empty(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+#if TEST_STD_VER >= 26
+  string.subview(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+#endif
 }

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

@@ -12,7 +12,12 @@
 
 #include <string_view>
 
+#include "test_macros.h"
+
 void test() {
   std::string_view string_view;
   string_view.empty(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+#if TEST_STD_VER >= 26
+  string_view.subview(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+#endif
 }

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

@@ -60,6 +60,10 @@
 #    error "__cpp_lib_string_resize_and_overwrite should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_string_udls
 #    error "__cpp_lib_string_udls should not be defined before c++14"
 #  endif
@@ -114,6 +118,10 @@
 #    error "__cpp_lib_string_resize_and_overwrite should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++26"
+#  endif
+
 #  ifndef __cpp_lib_string_udls
 #    error "__cpp_lib_string_udls should be defined in c++14"
 #  endif
@@ -177,6 +185,10 @@
 #    error "__cpp_lib_string_resize_and_overwrite should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++26"
+#  endif
+
 #  ifndef __cpp_lib_string_udls
 #    error "__cpp_lib_string_udls should be defined in c++17"
 #  endif
@@ -261,6 +273,10 @@
 #    error "__cpp_lib_string_resize_and_overwrite should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++26"
+#  endif
+
 #  ifndef __cpp_lib_string_udls
 #    error "__cpp_lib_string_udls should be defined in c++20"
 #  endif
@@ -354,6 +370,10 @@
 #    error "__cpp_lib_string_resize_and_overwrite should have the value 202110L in c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++26"
+#  endif
+
 #  ifndef __cpp_lib_string_udls
 #    error "__cpp_lib_string_udls should be defined in c++23"
 #  endif
@@ -456,6 +476,13 @@
 #    error "__cpp_lib_string_resize_and_overwrite should have the value 202110L in c++26"
 #  endif
 
+#  ifndef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should be defined in c++26"
+#  endif
+#  if __cpp_lib_string_subview != 202506L
+#    error "__cpp_lib_string_subview should have the value 202506L in c++26"
+#  endif
+
 #  ifndef __cpp_lib_string_udls
 #    error "__cpp_lib_string_udls should be defined in c++26"
 #  endif

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

@@ -40,6 +40,10 @@
 #    error "__cpp_lib_string_contains should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_string_view
 #    error "__cpp_lib_string_view should not be defined before c++17"
 #  endif
@@ -66,6 +70,10 @@
 #    error "__cpp_lib_string_contains should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_string_view
 #    error "__cpp_lib_string_view should not be defined before c++17"
 #  endif
@@ -92,6 +100,10 @@
 #    error "__cpp_lib_string_contains should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++26"
+#  endif
+
 #  ifndef __cpp_lib_string_view
 #    error "__cpp_lib_string_view should be defined in c++17"
 #  endif
@@ -136,6 +148,10 @@
 #    error "__cpp_lib_string_contains should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++26"
+#  endif
+
 #  ifndef __cpp_lib_string_view
 #    error "__cpp_lib_string_view should be defined in c++20"
 #  endif
@@ -183,6 +199,10 @@
 #    error "__cpp_lib_string_contains should have the value 202011L in c++23"
 #  endif
 
+#  ifdef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should not be defined before c++26"
+#  endif
+
 #  ifndef __cpp_lib_string_view
 #    error "__cpp_lib_string_view should be defined in c++23"
 #  endif
@@ -239,6 +259,13 @@
 #    error "__cpp_lib_string_contains should have the value 202011L in c++26"
 #  endif
 
+#  ifndef __cpp_lib_string_subview
+#    error "__cpp_lib_string_subview should be defined in c++26"
+#  endif
+#  if __cpp_lib_string_subview != 202506L
+#    error "__cpp_lib_string_subview should have the value 202506L in c++26"
+#  endif
+
 #  ifndef __cpp_lib_string_view
 #    error "__cpp_lib_string_view should be defined in c++26"
 #  endif