Improve apparent type of mapped types with mapped type modifiers #58091
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typescript-bot
added
the
For Uncommitted Bug
Andarist
commented
Apr 5, 2024
| const target = (type.target ?? type) as MappedType; | ||
| const typeVariable = getHomomorphicTypeVariable(target); | ||
| if (typeVariable && !target.declaration.nameType) { | ||
| const constraint = getConstraintTypeFromMappedType(type); | ||
| if (constraint.flags & TypeFlags.Index) { | ||
| const baseConstraint = getBaseConstraintOfType((constraint as IndexType).type); | ||
| if (baseConstraint && everyType(baseConstraint, t => isArrayOrTupleType(t) || isArrayOrTupleOrIntersection(t))) { | ||
| return instantiateType(target, prependTypeMapping(typeVariable, baseConstraint, type.mapper)); |
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The type.mapper here looks something like:
m1: T_id -> Box<string>[]
m2: m1: K -> K
m2: m1: T_id -> UnboxArray<T_outer>
m2: T_id -> UnboxArray<T_outer>Since typeVariable is T_id TS ended up "skipping" over the inner mapping of UnboxArray. The constraint (of the type) is keyof T_outer with a baseConstraint of Box<string>[].
In a sense, I'd like to map T_outer to baseConstraint (Box<string>[]), process that through type.mapper and then map T_id to that result (or smth along those lines). I couldn't find a clear way to do that given that we need to account for non-homomorphic instantiations of homomorphic mapped types here. Using the modifiers type does seem to do the trick but I'm not overly confident in this solution and I won't mind being proven that this is wrong 😉
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@typescript-bot test it |
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Hey @jakebailey, the results of running the DT tests are ready. Everything looks the same! |
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@jakebailey Here are the results of running the user tests comparing Everything looks good! |
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@jakebailey Here they are:
tscComparison Report - baseline..pr
System info unknown
Hosts
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Developer Information: |
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@jakebailey Here are the results of running the top 400 repos comparing Everything looks good! |
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Damn, I was close! :P It was worth a shot :) |
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fixes #58060