v4.7 and above stopped supporting a type pattern that used to work in v4.6 (Key Remapping in Mapped Type)

Bug Report

In version 4.7 and above a patter ([P in TName as P]:) causes types to break. It’s really strange, since it does work if used in some ways, but if used to pass down to a method argument, it will result in type unknown.

See playground links for examples.

🕗 Version & Regression Information

A friendly person in the comments of my original stackoverflow question comments narrowed it down to this: between 2022-03-15 and 2022-03-16

⏯ Playground Link

TS 4.7 TS 4.6

💻 Code

The issue was caused by the following types:

[P in TName as P]:

which does not work anymore in v4.7 and needs to be

[P in TName]:

(I had it like this, because I used to exclude a type like this: [P in TName as P extends UnacceptedPaths ? never : P]:)

🙁 Actual behavior

One of the nested types receives type unknown to it’s params, when it should be properly inferred from the code.

🙂 Expected behavior

Should work like in v4.6 - type should be inferred. If I remember correctly, I came up with the problematic type from this.

[RyanCavanaugh]

@ahejlsberg here’s a simplified version that presents as an implicit any regression:

type UnacceptedPaths = "really_bad" | "notgood";

type FieldsProp<TFieldValues, TName extends keyof TFieldValues> = {
    [P in TName as P extends UnacceptedPaths ? never : P]: (x: TFieldValues[P]) => void;
};

type RegisterFieldProps<TFieldValues> = {
    control: TFieldValues;
    fields: FieldsProp<TFieldValues, keyof TFieldValues>;
};

declare function RegisterFields<TFieldValues>(fields: RegisterFieldProps<TFieldValues>): unknown;
const TEST = RegisterFields({
    control: {
        test: 99,
    },
    fields: {
        test: (newVal1) => {
            // newVal1 should be type number (4.6)
            // actual: implicit any (4.8+)
            const a: number = newVal1;
        }
    },
});

Can we tweak back the originating PR to still allow inferences from mapped types where the as clause is of the form [K in T as expr ? K : never] / [K in T as expr ? never : K] ?

[MartinJohns]

Clarification, because I got confused: The issue only appears when you remove this line from the 4.7 playground:

simpleValue: "type_string_not_allowed", // funnily enough, this works properly