Type inference not working as expected when using key remapping in mapped types

Bug Report

🔎 Search Terms

mapped types, key remapping using as, type inference

🕗 Version & Regression Information

I tested this with all versions available on typescriptlang.org/play including nightly. I was unable to test this on versions < 4.1 because key-remapping does not exist prior to version 4.1.

⏯ Playground Link

Playground link with relevant code

💻 Code

// simple Maybe implementation for testing:
type Some<T> = {__kind: "Some", boxed: T}
type None = {__kind: "None"}
type Maybe<T> = Some<T> | None
function Some<T>(value: T): Maybe<T> {return {__kind: "Some", boxed: value}}
function None(): Maybe<unknown> {return {__kind: "None"}}


type MatchCases<T extends {__kind: K}, K extends string, R> = {[V in T as V["__kind"]]: (variant: V) => R}

function match<T extends {__kind: K}, K extends string, R>(variant: T, cases: MatchCases<T, K, R>): R {
    const _case = cases[variant.__kind]
    // _case is expected to receive an argument of type K instead of V
    return _case(variant)
}

// Example 1
// Declaring types explicitly works fine, the argument type of Some handler is inferred correctly.
// I intentionally use 'string' and 'unknown' instead of "None | Some" and 'number'
// to reflect the inferred types from below:
const withExplicitTypes = match<Maybe<number>, string, unknown>(Some(5), {
    None: () => 1,
    Some: ({boxed}) => boxed * 2
})

// Example 2
// The compiler infers the exact same types as above:
const withTypeInference = match(Some(5), {
    None: () => 1,
    // But the argument to Some-handler cannot be inferred correctly:
    Some: some => some.boxed 
})

🙁 Actual behavior

I might be misusing the key remapping feature, but while implementing the above mentioned match function, I ran into following behavior:

• calling match above with explicit generic types works as expected (Example 1)
• the compiler infers the exact same types when not provided explicitly, but the arguments to the case handlers are inferred as any (Example 2), even that intellisense initially proposes the correct type while typing (see [1])
• when trying to call a case function in the body of match, the expected type of the argument is K instead of T.

🙂 Expected behavior

• I'm expecting _case to require an argument of type V as declared in the mapped type ((variant: V) => R).
• Based on the inferred types of MatchCases, the argument of the Some-handler should be inferred to type Some<number>.

[1]:

Start with the linked TypeScript Playground and the provided MatchCases and match examples to reproduce the inference failure. Trace key remapping in mapped types and generic inference; done when the Some handler receives Some and the _case call accepts the mapped variant type rather than K.