Repository navigation
3.5 regression concerning type inference for non-inferable types #31735
Description
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.
Activity
ahejlsberg commentedon Jun 3, 2019
This is an effect of #30637, a known breaking change. In your example we make no inferences for the
Propstype parameter in the call tocreateStyles. This means we default tounknown, but since that doesn't satisfy the constraintobject, we pickobject. Previously we'd default to{}which did satisfy the constraint.eps1lon commentedon Jun 4, 2019
Suspected as much though I wasn't aware how this interacts with non-inferrable types. Thanks for the explanation.