TypeScript conditional types constrain no longer working after v3.5.1

[weichensw]

TypeScript Version: versions after 3.5.1, tested up to 4.1.0-dev.20200807

Search Terms: conditional constrain never

Code

type StringConstrain<T> = { [K in keyof T]: T[K] extends string ? T[K] : never }
type WantString<T extends string> = number
type Test<T extends StringConstrain<T>> = { [K in keyof T]: WantString<T[K]> }

Expected behavior:

The constrain in line 1 should function correctly and demands all keys of T to extend string in line 3, as such WantString<T[K]> should be happy.

Actual behavior:

In any version greater and including 3.5.1 line 3 is not happy

Type 'T[K]' does not satisfy the constraint 'string'.
  Type 'T[keyof T]' is not assignable to type 'string'.
    Type 'T[string] | T[number] | T[symbol]' is not assignable to type 'string'.
      Type 'T[string]' is not assignable to type 'string'.(2344)

I don't understand the new behavior and if this is not a bug, what's the intended way of achieving the same type constrain after 3.5.1?

Playground Link:

Playground link showing works in v3.3.3

Playground link showing not working in 3.9.2

Playground link showing not working in 4.1.0-dev.20200807

Related Issues:

Same question on stackoverflow from last month: https://stackoverflow.com/questions/62563003/typescript-conditional-types-constrain-no-longer-working-after-v3-5-1

[jack-williams]

Duplicate of #32550. It was probably working in <3.5.1 for the wrong reasons.

Does this work for you? type Test<T extends Record<keyof T, string>> = { [K in keyof T]: WantString<T[K]> }

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.