3.5 regression concerning type inference for non-inferable types

[eps1lon]

TypeScript Version: typescript@3.6.0-dev.20190603

Search Terms:
3.5, infer

Code

// mocked CssProperties type
type CssProperties = { position: "absolute" | "relative" };

type StylesObject<ClassKey extends string> = Record<ClassKey, CssProperties>;
type PropsBasedstyles<ClassKey extends string, Props extends object> = (
  props: Props
) => StylesObject<ClassKey>;
type Styles<ClassKey extends string, Props extends object> =
  | StylesObject<ClassKey>
  | PropsBasedstyles<ClassKey, Props>;

function createStyles<ClassKey extends string, Props extends object>(
  styles: Styles<ClassKey, Props>
): Styles<ClassKey, Props> {
  return styles;
}

const styles = createStyles({
  root: {
    position: "absolute"
  }
});

Expected behavior:
Behavior in 3.4.5
typeof styles = Styles<'root', {}>

Actual behavior:
Behavior in >=3.5.0
typeof styles = Styles<'root', object>

Playground Link:
playground

Related Issues: Did not find any

Notes:

Seems like the playground is still using 3.4.

Usage of this behavior is showcased in https://github.com/eps1lon/ts-inference-3.5/blob/master/index.ts with a fix for 3.5.

[ahejlsberg]

This is an effect of #30637, a known breaking change. In your example we make no inferences for the Props type parameter in the call to createStyles. This means we default to unknown, but since that doesn't satisfy the constraint object, we pick object. Previously we'd default to {} which did satisfy the constraint.

[eps1lon]

Suspected as much though I wasn't aware how this interacts with non-inferrable types. Thanks for the explanation.