Type alias substitution does not work

[reverofevil]

TypeScript Version: 3.5.0-dev.20190525

Search Terms:
type substitution is incorrect

Code

const v = { test: { smth: 5 } };
type Field<A extends string, R> = { [K in A]: R } | {}
const f = <A extends string, B extends string, R>(x: { [K in A]: Field<B, R> } | {}): R => ({} as any);
const g = <A extends string, B extends string, R>(x: Field<A, Field<B, R>>): R => ({} as any);
const r1 = f(v); // number
const r2 = g(v); // unknown

Expected behavior:
Both should be number.

Actual behavior:
unknown

Playground Link:
Link

Related Issues:
Nope.

[RyanCavanaugh]

Regressed between 2.3 and 2.4

Removed suspect | {}:

const v = { test: { smth: 5 } };
type Field<A extends string, R> = { [K in A]: R };
const f = <A extends string, B extends string, R>(x: { [K in A]: Field<B, R> } ): R => ({} as any);
const g = <A extends string, B extends string, R>(x: Field<A, Field<B, R>>): R => ({} as any);
const r1 = f(v); // number
const r2 = g(v); // unknown

[weswigham]

Regression caused by #15863 - potential fix is up at #31633 - I need to run RWC to validate fp-ts doesn't OOM (which was what originally prompted the original change that caused the regression), since the change introducing the regression didn't come with any baked-in tests ❤️

[RyanCavanaugh]

These are both number now as expected