Conditional types don't distribute union types with property access

[falsandtru]

cc @ahejlsberg

TypeScript Version: 2.7.0-dev.20180327

Search Terms:

Code

type Id<T> = T;
type Z<T> = Id<T> extends true ? Id<T> : never;
type z = Z<boolean>;
type A<T extends any[]> = T[0] extends true ? T[0] : never;
type a = A<[boolean]>;
type B<T extends {}> = { [P in keyof T]: T[P] extends true ? T[P] : never }[keyof T];
type b = B<[boolean]>;

Expected behavior:

z, a, b is true.

Actual behavior:

z is true but a, b is never.

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Related Issues:

[ahejlsberg]

The conditional type in Z is distributive, but the conditional types in A and B are not, so this is behaving as expected. Since type aliases are equivalent to writing the expansion inline, the checked type in Z is a naked type parameter and thus the conditional is distributive.

[falsandtru]

I see, thanks.