[libc++][ranges] Implement the changes to `basic_string` from P1206 (`ranges::to`):

- add the `from_range_t` constructors and the related deduction guides;
- add the `insert_range`/`assign_range`/etc. member functions.

(Note: this patch is split from https://reviews.llvm.org/D142335)

Differential Revision: https://reviews.llvm.org/D149832

1 files changed, 50 insertions, 0 deletions

diff --git a/libcxx/test/std/strings/basic.string/string.cons/from_range_deduction.pass.cpp b/libcxx/test/std/strings/basic.string/string.cons/from_range_deduction.pass.cpp
new file mode 100644
index 000000000000..4df4d39c2355
--- /dev/null
+++ b/libcxx/test/std/strings/basic.string/string.cons/from_range_deduction.pass.cpp
@@ -0,0 +1,50 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <string>
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+// To silence a GCC warning-turned-error re. `BadAlloc::value_type`.
+// ADDITIONAL_COMPILE_FLAGS: -Wno-unused-local-typedefs
+
+// template<ranges::input_range R,
+//          class Allocator = allocator<ranges::range_value_t<R>>>
+//   basic_string(from_range_t, R&&, Allocator = Allocator())
+//     -> basic_string<ranges::range_value_t<R>, char_traits<ranges::range_value_t<R>>,
+//                     Allocator>; // C++23
+//
+// The deduction guide shall not participate in overload resolution if Allocator
+// is a type that does not qualify as an allocator (in addition to the `input_range` concept being satisfied by `R`).
+
+#include <array>
+#include <string>
+
+#include "deduction_guides_sfinae_checks.h"
+#include "test_allocator.h"
+
+int main(int, char**) {
+  using Char = char16_t;
+
+  {
+    std::basic_string c(std::from_range, std::array<Char, 0>());
+    static_assert(std::is_same_v<decltype(c), std::basic_string<Char>>);
+  }
+
+  {
+    using Alloc = test_allocator<Char>;
+    std::basic_string c(std::from_range, std::array<Char, 0>(), Alloc());
+    static_assert(std::is_same_v<decltype(c), std::basic_string<Char, std::char_traits<Char>, Alloc>>);
+  }
+
+  // Note: defining `value_type` is a workaround because one of the deduction guides will end up instantiating
+  // `basic_string`, and that would fail with a hard error if the given allocator doesn't define `value_type`.
+  struct BadAlloc { using value_type = char; };
+  SequenceContainerDeductionGuidesSfinaeAway<std::basic_string, std::basic_string<char>, BadAlloc>();
+
+  return 0;
+}