No return type validation for function that returns mapped object with template literals as keys

[kbialowas]

Bug Report

🔎 Search Terms

"return type validation"
"mapped type resolved to any"
"template literals"

🕗 Version & Regression Information

Happens on every version I tried since 4.1.5 (previous versions says, that operator 'as' isn't available).

⏯ Playground Link

Playground link with relevant code

💻 Code

type MappedKeys<T extends string, TBase={bar: unknown, baz: unknown}> = {
    [key in keyof TBase as `${T}_${string & key}`]: number
}
function mapKeys<T extends string>(prefix: T): MappedKeys<T> {
    return "No type controll :("
}
const a = mapKeys("foo");
a.foo_bar

🙁 Actual behavior

Typescript shows no error for mapKeys function return type.

Typing a. suggests that a is an object and has two fields - a.foo_bar and a.foo_baz.

🙂 Expected behavior

Typescript compiler should show error when returned type isn't matching declared return type.

[kbialowas]

I guess it's related to limitations mentioned here:

#23918
#23433

When I remove generic prefix, checks work fine.