T[keyof T] type constraint lost in function body

TypeScript Version: 2.1.4 / nightly (2.2.0-dev.20161216)

function fn<T extends {[P in K]: string}, K extends keyof T>(obj: T, key: K) {
  const val: string = obj[key];  // error TS2322: Type 'T[K]' is not assignable to type 'string'
}

fn({ str: "s", num: 1 }, "str"); // OK
fn({ str: "s", num: 1 }, "num"); // error TS2345: Argument of type '{ str: string; num: number; }' is not assignable to parameter of type '{ num: string; }

Expected behavior:

Given the constraints, I would expect obj[key] in this case to be of type string.

Actual behavior:

The type of obj[key] is the less specific T[K].

Considering the error at the second call site (which is expected), it seems that the type checking done there is complete and working. However, that same type info is not carried through into the function body itself.

I witnessed the same bug. I noticed that this problem only exists if both obj and key are TypeParameters.

// typescript@2.7.0-rc with --strictNullChecks
declare function foo(p: string): void;

interface I {
    foo?: string;
    bar?: string;
}

function working1<T extends I>(o: T, k: keyof I) {
    foo(o[k]); // getting expected type error here, because type is `string | undefined`
}

function working2<K extends keyof I>(o: I, k: K) {
    foo(o[k]); // getting expected type error here, because type is `string | undefined`
}

function notWorking<T extends I, K extends keyof T>(o: T, k: K) {
    foo(o[k]); // expected type error, but got none
}

Inside notWorking o[k] is treated as any. At least that's what checker.getApparentType(checker.getTypeAtLocation(node)) tells me.

These should be working now as expected.

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T[keyof T] type constraint lost in function body #12991

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T[keyof T] type constraint lost in function body #12991

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ajafff commented on Jan 23, 2018

mhegazy commented on Feb 1, 2018

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