Fixes and code cleanup for include/pfn/expected.hpp

[Bronek]

Changes summary:

• Constraint on converting assignment wrongly used is_assignable_v<T, U>, SFINAE-disabling the template for scalar T. Fixed and tightened noexcept extension in the same.
• unexpected::operator== now uses error() instead of the private member, fixing heterogeneous comparison. Added a noexcept extension in the same.
• Added/improved comments, added reference to P3798 has_error()
• Improved unit tests coverage, fixed typos in test names
• Removed dead code _can_convert in expected<void, E>

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
include/pfn/expected.hpp	71.4%	6 Missing ⚠️

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[augmentcode]

🤖 Augment PR Summary

Summary: Cleans up and fixes the pfn::expected polyfill implementation, with an emphasis on correct constraints and improved noexcept propagation (as an explicit extension).

Changes:

• Added a header-level note documenting the polyfill’s intent and the project’s “derived noexcept” extension policy.
• Fixed converting assignment constraints by using std::is_assignable_v<T&, U> (avoids unintended SFINAE for scalar T).
• Updated unexpected::operator== to compare via error() (enables heterogeneous comparisons) and extended its noexcept per the extension policy.
• Tightened expected conversion-assignment noexcept clauses to reflect assign/construct properties more directly.
• Annotated has_error() with a reference to P3798.
• Removed unused/dead internal helper code in expected<void, E>.
• Expanded unit tests for heterogeneous unexpected equality and for assignment/swap noexcept behavior across more type combinations.
• Enhanced helper test types to model throwing assignment operators (used to validate propagation behavior).

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[augmentcode]

Review completed. 1 suggestion posted.

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[augmentcode]

include/pfn/expected.hpp:149 — The noexcept computation uses static_cast<bool>(x.error() == y.error()); this can succeed even when the comparison result is only explicitly convertible to bool, while the return statement still requires an implicit conversion to bool. Consider aligning the noexcept/participation check with implicit convertibility to avoid surprising hard errors in some generic contexts.

Severity: low

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[sonarqubecloud]

Quality Gate failed

Failed conditions
19.4% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud