[Bronek]

A non-throwing callback can return an optional or expected by reference while copying or moving that result throws. The monadic operation previously overlooked that construction in its exception specification and could terminate instead of propagating the exception.

Account for result construction in and_then and or_else for owning and reference pfn::optional and both pfn::expected specializations. Use is_nothrow_invocable_r while retaining the existing checks for constructing the unchanged alternative. Preserve guaranteed copy elision for immovable prvalues, the draft's constraints, and invalid-callback diagnostics.

Addresses the pfn portion of #436. No include/fn files are changed; the fn wrapper fixes remain separate work. Document that remaining difference and exclude these new regressions from the shared suite's fn instantiations.

Validation:

• Full GCC and Clang builds with libstdc++: 89/89 CTests passed on each.
• C++23 and C++26 standard-library validation translation units compiled for optional and expected on both compilers.
• Regressions cover throwing copies and moves, source and result cv/ref categories, non-throwing controls, and immovable prvalues. They fail against the preceding implementation.
• Claude reviewed the fixes; test-portability and documentation feedback is incorporated. Pre-commit checks passed.

Assisted-by: Codex:gpt-6

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[augmentcode]

🤖 Augment PR Summary

Summary: Corrects callback-result handling in pfn monadic exception specifications.

• Updates and_then and or_else to use is_nothrow_invocable_r.
• Covers owning and reference optional specializations.
• Covers object and void expected specializations.
• Allows throwing copies or moves of reference-returned results to propagate rather than terminate.
• Retains checks for constructing unchanged values and errors.
• Preserves guaranteed copy elision for immovable prvalue results.
• Leaves participation constraints and invalid-callback diagnostics unchanged.
• Adds runtime and constexpr regressions for cv/ref categories, throwing construction, and non-throwing controls.
• Documents the remaining fn wrapper issue (and_then overlooks the copy when a callback returns a carrier by reference #436) and excludes these regressions from its shared-suite instantiations.

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[augmentcode]

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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