Higher order type relations for mapped types#12351
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Higher order type relations for mapped types#12351
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mhegazy
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Higher order type relations for mapped types
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This PR adds a number of type relations for higher order mapped types and indexed access types (i.e. un-instantiated generic forms of the types).
S[K]is related to a typeT[K]ifSis related toT.Tis related to a type{ [P in keyof T]: X }ifT[P]is related toX.{ [P in keyof T]: X }is related to a typeTifXis related toT[P].The PR furthermore adds the following rules:
keyof { [P in K]: X }is equivalent to justK. For example,keyof Partial<T>is equivalent tokeyof T.{ [P in K]: T}[X]is equivalent to an instantiation ofTwhereXis substituted for every occurrence ofP. For example,{ [P in K]: Box<T[P]> }[X]is equivalent toBox<T[X]>.type T = { [P in K]: X }it is an error forKto directly or indirectly referenceT.Twith a constraint that includes a string index signature of typeX, the apparent type of an indexed access typeT[K]isX.Fixes #12311.
Fixes #12315.