Constrain the callback of the sum-case transform and transform_error#297
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This was referenced Jul 13, 2026
🤖 Augment PR SummarySummary: Tightens the immediate-context constraints for sum-delegating monadic statics so invalid callbacks are rejected via constraints (dropping non-viable overloads) rather than failing during deduced-return body instantiation. Changes:
Technical Notes: This addresses overload-resolution poisoning and hard errors from failures discovered only inside deduced-return bodies, aligning behavior with other monadic helpers and pfn compatibility. 🤖 Was this summary useful? React with 👍 or 👎 |
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The sum-delegating statics constrained only the grade, leaving callback validity to be discovered inside the deduced-return body. Two consequences, both outside the immediate context: a callback no alternative could take was a hard error where the non-sum path cleanly drops the candidate, and overload resolution was poisoned - the losing `const &` candidate formed its signature too, so a visitor serving only the category the call selects made the call ill-formed. Constraining the callback in the immediate context, as `sum::transform` itself does, means a partial visitor is now enough. The existing tests spell all four value categories in every visitor because they had to; the new blocks assert they no longer do. Closes #277 Assisted-by: Claude:claude-opus-4-8[1m]
The body puts the untouched value into the result, but only the noexcept spec weighed that copy - the requires-clause did not, alone among the monadic statics (`_or_else`, `_and_then` and `_transform` all carry their untouched-side conjunct). The copy was therefore checked while instantiating the deduced-return body, and since `const &` binds rvalues, the losing candidate hard-errored for a move-only value type: `fn::expected<move_only, E>` could not call `transform_error` from any value category, where `pfn::expected` supports the rvalue call and cleanly drops the lvalue one. A pfn -> fn switch turned valid code ill-formed. Closes #278 Assisted-by: Claude:claude-opus-4-8[1m]
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Take #297's new constraint sections for the sum-case transform and transform_error, and retire the GAP notes they answer. Both defects were hard errors, so the suite could only describe them in comments: a callback no alternative accepts now drops the candidate, a visitor need only serve the category the call selects, and a move-only value drops the copying overloads. Main's blocks land as SECTIONs, keeping the suite free of the BDD macros. Assisted-by: Claude:claude-opus-4-8[1m]
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Two missing conjuncts in the constraints of the monadic statics, both leaving a failure to be discovered inside the deduced-return body instead of the immediate context.
Constrain the callback of the sum-case
transformandtransform_errorThe sum-delegating statics constrained only the grade, leaving callback validity to be discovered inside the deduced-return body. Two consequences, both outside the immediate context: a callback no alternative could take was a hard error where the non-sum path cleanly drops the candidate, and overload resolution was poisoned - the losing
const &candidate formed its signature too, so a visitor serving only the category the call selects made the call ill-formed.Constraining the callback in the immediate context, as
sum::transformitself does, means a partial visitor is now enough. The existing tests spell all four value categories in every visitor because they had to; the new blocks assert they no longer do.Require of
transform_errorthat the untouched value can be relocatedThe body puts the untouched value into the result, but only the noexcept spec weighed that copy - the requires-clause did not, alone among the monadic statics (
_or_else,_and_thenand_transformall carry their untouched-side conjunct). The copy was therefore checked while instantiating the deduced-return body, and sinceconst &binds rvalues, the losing candidate hard-errored for a move-only value type:fn::expected<move_only, E>could not calltransform_errorfrom any value category, wherepfn::expectedsupports the rvalue call and cleanly drops the lvalue one. A pfn -> fn switch turned valid code ill-formed.Closes #277
Closes #278
Stack: P3 of 13 — the release-0.1 bugfix queue, merging bottom-up into
main. Based on #296 and to be retargeted tomainonce it merges; the diff shows only this PR's commits. Every stack boundary was validated with a gcc build and the full test suite on top ofmain, and the aggregate branch ran the full CI matrix green (#289).