fix: dispatch on whether a comparator is implemented - #2840
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zachdaniel merged 2 commits intoAug 4, 2026
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`Comp` cannot dispatch a protocol on two operand types directly, so it encodes the pair into a synthetic module name and dispatches on that. Nothing today pins which implementation a given pair actually reaches, so a change to how that name is resolved could route every pair to the generic fallback without a single test noticing. Pin both outcomes with comparisons the fallback would answer differently: Erlang term order is case-sensitive, orders a struct before a binary, and orders `Decimal` by its fields rather than by its value. Passes against unmodified code.
`Module.safe_concat/1` succeeds when the atom exists, which says nothing about
whether the pair has a comparator behind it. Anything that ever names
`Comparable.Type.<L>.To.<R>` creates that atom — a dynamic `Module.concat/1`, a
typespec, a doc reference — so the same comparison answers from the fallback or
raises `Protocol.UndefinedError` depending on ambient state the caller can
neither see nor control:
Comp.compare(Ash.CiString.new("a"), Decimal.new(1))
#=> :lt
_ = Comparable.Type.Ash.CiString.To.Decimal
Comp.compare(Ash.CiString.new("a"), Decimal.new(1))
#=> ** (Protocol.UndefinedError)
Decide on the implementation instead, which is the condition the rescue was
reaching for. Pairs that do have a comparator still reach it, pinned by the
preceding commit.
This makes the answer consistent, not meaningful: the fallback still orders
`Ash.CiString` against `Decimal` by map size rather than by anything about the
values. `Comparable.compare/1` returns `:lt | :eq | :gt` or raises, so there is
no way to say a pair has no ordering.
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Summary
Fixes #2839