Conditional types fail to distribute in properties of mapped types

TypeScript Version: 3.7.0-dev.20190928

Search Terms: conditional mapped property union

Code

type NullifyStrings<T> = T extends string ? null : T
type NullifyStringsInPropsWorking<T> = { [K in keyof T]: NullifyStrings<T[K]> }
type NullifyStringsInPropsBroken<T> = { [K in keyof T]: T[K] extends string ? null : T[K] }

type TestType = { a: number | string }
// { a: number | null }
type WorkingReplaceProps = NullifyStringsInPropsWorking<TestType>
// { a: string | number }
type BrokenReplaceProps = NullifyStringsInPropsBroken<TestType>

Expected behavior:
NullifyStringsInPropsWorking and NullifyStringsInPropsBroken should be functionally identical - expanding the NullifyStrings type alias in NullifyStringsInPropsWorking results in the same definition as NullifyStringsInPropsBroken.

Actual behavior:
NullifyStringsInPropsWorking and NullifyStringsInPropsBroken have different behaviour - BrokenReplaceProps has type { a: string | number } instead of the expected { a: number | null }.

Playground Link: Link

Related Issues:

#28339, but seems to be different.

#22945 mentions

Since type aliases are equivalent to writing the expansion inline [...]

but that is not the case here.

Inizia con gli esempi minimi di conditional e mapped-type nell’issue e riproducili nel TypeScript Playground collegato. Leggi le issue correlate #28339 e #22945 per il contesto, quindi traccia il comportamento del compilatore responsabile della differenza tra i due alias. Il lavoro è completo quando entrambi gli alias producono il risultato atteso { a: number | null } e viene aggiunta la copertura delle regressioni nei test rilevanti del compilatore.