Add assignability rule relaxing the assignability of partial mapped types #30112
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ahejlsberg
reviewed
Mar 8, 2019
src/compiler/checker.ts
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| const hasOptionalUnionKeys = modifiers & MappedTypeModifiers.IncludeOptional && targetConstraint.flags & TypeFlags.Union; | ||
| const filteredByApplicability = hasOptionalUnionKeys ? filterType(targetConstraint, t => !!isRelatedTo(t, sourceKeys)) : undefined; | ||
| // A source type T is related to a target type { [P in Q]: X } if Q is related to keyof T and T[Q] is related to X. | ||
| // A source type T is related to a target type { [P in Q]?: X } if some Q = Q' is related to keyof T and T[Q'] is related to X. |
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| // A source type T is related to a target type { [P in Q]?: X } if some Q = Q' is related to keyof T and T[Q'] is related to X. | |
| // A source type T is related to a target type { [P in Q]?: X } if some constituent Q' of Q is related to keyof T and T[Q'] is related to X. |
ahejlsberg
approved these changes
Mar 8, 2019
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Fixes #30081
The usual rule right now is that all the keys in the target mapped type's constraint must be assignable to the source's keys. With this change, for partial mapped types, only some key of the partial mapped type must be assignable to the source's keys (and then only with those keys do we index into the source to compare values).