Implement LWG-4259 P1148R0 changed the return values of searching functions of std::basic_string on some platforms

[YexuanXiao]

Fixes #6208.

[Copilot]

Pull request overview

Implements the LWG-4259/P1148R0 behavior change so std::basic_string searching functions return basic_string::npos (not a truncated size_t(-1)/string_view::npos) when basic_string::size_type is wider than size_t, and adds a regression test for that scenario.

Changes:

• Add an internal helper to cast search results from size_t to basic_string::size_type, mapping size_t(-1) to basic_string::npos when needed.
• Update all basic_string search-family functions (find*/rfind*) to use the helper.
• Add a test using a custom allocator with a wider size_type to validate npos return values.

Show a summary per file

File	Description
tests/std/tests/GH_005546_containers_size_type_cast/test.cpp	Adds a wide-size_type allocator + new test cases covering basic_string search functions returning npos.
stl/inc/xstring	Fixes return-value casting for basic_string searching functions when size_type is wider than size_t.

Copilot's findings

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• Files reviewed: 2/2 changed files
• Comments generated: 3

[Copilot]

WideSizeAllocator::allocate/deallocate are declared constexpr unconditionally, but they call std::allocator<T>::allocate/deallocate, which isn't constexpr before C++20. This can make pre-C++20 builds ill-formed even if these functions are never used in a constant expression. Use the existing CONSTEXPR20 macro for these member functions (and consider calling check_alloc_in_range_of_size_t(n) before the size_t cast to avoid silent truncation, matching redifference_allocator).

Suggested change

	constexpr T* allocate(size_type n) {
	return allocator<T>{}.allocate(static_cast<size_t>(n));
	}
	constexpr void deallocate(T* p, size_type n) noexcept {
	CONSTEXPR20 T* allocate(size_type n) {
	check_alloc_in_range_of_size_t(n);
	return allocator<T>{}.allocate(static_cast<size_t>(n));
	}
	CONSTEXPR20 void deallocate(T* p, size_type n) noexcept {
	check_alloc_in_range_of_size_t(n);

[Copilot]

WideSizeAllocator doesn't define allocator equality (operator==/!=). Allocators used with standard containers are expected to be equality comparable, and basic_string operations may instantiate allocator comparisons (e.g. swap/move assignment) even if this test currently doesn't. Consider adding the same style of friend constexpr bool operator== (and != when needed) used by redifference_allocator earlier in this file.

[Copilot]

_Find_result_cast takes a parameter named _Off, but it is the result from the internal _Traits_* search helpers, not an offset. Renaming the parameter (e.g. _Result/_Pos) would make the intent clearer and reduce the chance of accidental misuse when additional callers are added.

Suggested change

	_NODISCARD static _CONSTEXPR20 size_type _Find_result_cast(const size_t _Off) noexcept {
	if constexpr (sizeof(size_type) > sizeof(size_t)) { // only happens on 32-bit platforms with 64-bit size_type
	if (_Off == static_cast<size_t>(-1)) {
	return npos;
	}
	}
	return static_cast<size_type>(_Off);
	_NODISCARD static _CONSTEXPR20 size_type _Find_result_cast(const size_t _Result) noexcept {
	if constexpr (sizeof(size_type) > sizeof(size_t)) { // only happens on 32-bit platforms with 64-bit size_type
	if (_Result == static_cast<size_t>(-1)) {
	return npos;
	}
	}
	return static_cast<size_type>(_Result);