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

[Yexuan Xiao (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

[Stephan T. Lavavej (StephanTLavavej)]

Thanks! 😻 I pushed a bunch of nitpicky changes.

[Copilot]

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0 new
• Review effort level: Medium

[Stephan T. Lavavej (StephanTLavavej)]

I'm mirroring this to the MSVC-internal repo. Please notify me if any further changes are pushed, otherwise no action is required.

[Stephan T. Lavavej (StephanTLavavej)]

Thanks for implementing this LWG issue resolution, and apologies for how long it took to review this! 😹 🎉 🚀