The results of the 'Mapped Types' distribution have changed.

Bug Report

The conversion result of ‘Mapped Types’ has changed in 4.3.2 or later typescript.

💻 Code

type Base = { 100: { a: number }; 200: { b: string } }
type NewType<T = Base> = {
  [P in keyof T ]: {
    code: P
    body: T[P]
  }
} extends {
  [P in any]: infer R
}
  ? R
  : never

let test: NewType

🙁 Actual behavior

• 4.3.0-beta and earlier

let test: {
    code: 100;
    body: {
        a: number;
    };
} | {
    code: 200;
    body: {
        b: string;
    };
}

• 4.3.2

let test: {
    code: keyof Base;
    body: {
        a: number;
    } | {
        b: string;
    };
}

🙂 Expected behavior

Hoping to return to pre-4.3.0-beta behavior!

[vain0x]

Another workaround (tried on ts 4.4.0-dev.20210530):

type NewType<T> = {
  [P in keyof T]: {
    code: P
    body: T[P]
  }
}[keyof T]

type Base = { 100: { a: number }; 200: { b: string } }
declare const test: NewType<Base>

[weswigham]

I think our syntactic inference constraint rules are kiiiiiinda arbitrary to begin with (what type parameters we match up across what locations has kinda been chosen by request) - I “fixed” the issue by adding one because the change in behavior was technically a regression, and adding those syntactic constraints is sort-of free to do. The extra instantiation would probably be the best thing we could do to keep the old behavior going. We could back it out… But then we’d be reintroducing the old regression…