Fix ambiguous partial specialization when appending a sum to a pack#295
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`_pack_append`'s two specializations both matched a sum and neither subsumed the other, so naming `append_type<sum>` was ill-formed: gcc hard-errored even inside a requires-expression, while clang answered false. They now exclude each other, and the sum case defines no `type`, making the rejection a clean substitution failure on both compilers. Drops `_append<sum> = delete`, which `append`'s trailing return type made unreachable. Closes #282 Assisted-by: Claude:claude-opus-4-8[1m]
The tag selects the element type; it is never an element. With a type that has no default constructor the in-place overload dropped out on its `is_constructible_v` conjunct and the deduced-argument overload picked the call up, silently appending the tag itself, so `append(in_place_type<T>)` meant "construct a T" or "append the tag" depending on a property of T. The deduced-argument overloads now carry the guard `as_pack` already had, leaving the in-place overload as the only candidate for a tag. Closes #283 Assisted-by: Claude:claude-opus-4-8[1m]
This was referenced Jul 13, 2026
🤖 Augment PR SummarySummary: Fixes two constraint/overload-resolution defects in Changes:
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Bronek
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Take #295's corrected pack.append expectations into the restructured constraints section: it fixes #282 and #283, which this branch had pinned as GAP comments, and makes the sum negative probes portable at last. The #280 noexcept tripwires stay - that issue is still open. Assisted-by: Claude:claude-opus-4-8[1m]
Bronek
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The GAP note it replaces described an overload that no longer exists: #295 dropped `_append<sum> = delete` as unreachable, and made naming append_type<sum> a clean substitution failure on both compilers, which the note said no portable probe could ask. Assisted-by: Claude:claude-opus-4-8[1m]
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Two defects in the same
pack::appendoverload set.Fix ambiguous partial specialization when appending a
sumto apack_pack_append's two specializations both matched a sum and neither subsumed the other, so namingappend_type<sum>was ill-formed: gcc hard-errored even inside a requires-expression, while clang answered false. They now exclude each other, and the sum case defines notype, making the rejection a clean substitution failure on both compilers. Drops_append<sum> = delete, whichappend's trailing return type made unreachable.Reject
in_place_typeas a deducedpack::appendargumentThe tag selects the element type; it is never an element. With a type that has no default constructor the in-place overload dropped out on its
is_constructible_vconjunct and the deduced-argument overload picked the call up, silently appending the tag itself, soappend(in_place_type<T>)meant "construct a T" or "append the tag" depending on a property of T. The deduced-argument overloads now carry the guardas_packalready had, leaving the in-place overload as the only candidate for a tag.Closes #282
Closes #283
Stack: P1 of 13 — the release-0.1 bugfix queue, merging bottom-up into
main; this PR is the base of the stack. 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).