Exclude<T, U> type not working like Diff<T, U>

[yortus]

TypeScript Version: 2.8.0-dev.20180213

Search Terms:

• Exclude
• Diff

Code

// Old 'Diff' type, superseded since #21847 by new 'Exclude' builtin type
type Diff<T extends string, U extends string>
    = ({[P in T]: P } & {[P in U]: never } & { [x: string]: never })[T];

type NonFooKeys1<T extends object> = Diff<keyof T, 'foo'>;      // remove 'foo' using Diff
type NonFooKeys2<T extends object> = Exclude<keyof T, 'foo'>;   // remove 'foo' using Exclude

type Test1 = NonFooKeys1<{foo: 1, bar: 2, baz: 3}>; // Test1 is "bar" | "baz"
type Test2 = NonFooKeys2<{foo: 1, bar: 2, baz: 3}>; // Test2 is "foo" | "bar" | "baz"

Expected behavior:
Test2 should be "bar" | "baz", the same as Test1.

Actual behavior:
The key "foo" was not removed from keyof T when using the Exclude builtin type.

Related Issues:

• Predefined conditional types #21847
• Add Omit<T,K> and Diff<T,U> to lib.d.ts #19569

[DanielRosenwasser]

Of note, I'm getting some strange quick info. When defined as

type NonFooKeys<T> = Exclude<keyof T, 'foo'>;

I get

type NonFooKeys<T> = keyof T

[jack-williams]

Is this related to #21664 ? Exclude works when used directly for me.

type BarBaz = Exclude<keyof { foo: 1, bar: 2, baz: 3 }, 'foo'> // BarBaz = "bar" | "baz"

[falsandtru]

related #21862.

[ahejlsberg]

Duplicate of #21862.

[KiaraGrouwstra]

The other issue is marked fixed now.