Merged type not inferred as a mapped type argument

[dwickern]

TypeScript Version: 2.5.x, 2.6.0-dev.20170908

When passed as an argument, T & U evaluates to T, and the type U is discarded.

declare function f<T, Key extends keyof T>(obj: {[K in keyof T]: T[K]}, key: Key): T[Key];

declare const obj: { a: string } & { b: string };
f(obj, 'a');
f(obj, 'b'); // error

Playground link

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Workarounds

Specify the type arguments explicitly:

f<typeof obj, keyof typeof obj>(obj, 'b');

An identity type mapping can be used to create an intermediate type

type Fix<T> = {[P in keyof T]: T[P]}

declare const obj: Fix<{ a: string } & { b: string }>;
f(obj, 'a');
f(obj, 'b');

Related issue typed-ember/ember-typings#32

[ahejlsberg]

This is an issue with how we break down intersection types in type inference. Here's a simpler repro:

declare function f<T>(x: { prop: T }): T;

declare const a: { prop: string } & { prop: number };
declare const b: { prop: string & number };

f(a);  // string
f(b);  // string & number

We obviously shouldn't deliver different results above. Both should be string & number.

[dwickern]

Thanks for the quick fix. I can confirm this is working in typescript@2.6.0-dev.20170913