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Immediate keyof operation on intersection of generic mapped types incorrectly results in never type #22291
Description
TypeScript Version: 2.8.0-dev.20180302
Search Terms:
keyof, intersection, mapped type, generic, type parameter, never
Code
type Example1<T extends string, U extends string> = keyof (Record<T, any> & Record<U, any>)
type Result1 = Example1<'x', 'y'>
// Type of Result1 is never but it should be 'x' | 'y'
/** The additional examples below are just for context */
type Result2 = keyof (Record<'x', any> & Record<'y', any>)
// Type of Result2 is 'x' | 'y' as expected
type Example3<T extends string> = keyof (Record<T, any>)
type Result3 = Example3<'x' | 'y'>
// Type of Result3 is 'x' | 'y' as expected
type Example4<T extends string, U extends string> = (Record<T, any> & Record<U, any>)
type Result4 = keyof Example4<'x', 'y'>
// Type of Result4 is 'x' | 'y' as expected
type Example5<T, U> = keyof (T & U)
type Result5 = Example5<Record<'x', any>, Record<'y', any>>
// Type of Result5 is 'x' | 'y' as expectedExpected behavior:
Result1 should have a type of 'x' | 'y'
Actual behavior:
Result1 has a type of never
Playground Link:
Link
Related Issues:
Maybe #18538? I read through it but honestly I'm not clear on what is going on in that one so I can't say how related it is.
Activity
ahejlsberg commentedon Mar 2, 2018
The issue here is that for generic types
AandBwe should considerkeyof (A & B)to be equivalent tokeyof A | keyof B. We currently don't implement that transformation. If we did, we would transformkeyof (Record<T, any> & Record<U, any>)tokeyof Record<T, any> | keyof Record<U, any>, which we already reduce toT | U, the desired result.