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Regression in generic inference with JSX in v5.7+Β #62140
Description
π Search Terms
tsx generic
π Version & Regression Information
- This changed between versions 5.6.3 and 5.7.3
β― Playground Link
π» Code
import { defineProps as vueDefineProps } from 'vue';
declare const Comp: {
new <__VERTER__TS__T extends string = ''>(): {
$props: defineProps<__VERTER__TS__T>
};
}
(
<Comp name="foo"
onName={e => {
// @ts-expect-error this should never happen
if (e !== 'dd') {
}
}} />
);
; function TemplateBindingFN<T extends string>() {
const Props = vueDefineProps<{
name: T;
onName: (n: T) => void;
}>()
return { defineProps: Props as typeof Props }
};
export type defineProps<T extends string> = ReturnType<typeof TemplateBindingFN<T>>['defineProps']π Actual behavior
In later version the onName argument does not get the generic information resolved, defaulting to string, earlier versions 5.x infer e as 'foo'
π Expected behavior
e should be implicitly inferred based on the generic passed in name
Additional information about the issue
It seems to be caused by the type mutation of DefineProps
export type DefineProps<T, BKeys extends keyof T> = Readonly<T> & {
readonly [K in BKeys]-?: boolean
}
Activity
Andarist commentedon Jul 28, 2025
Bisects to #52095
Those types simplify to:
Since that PR, both intersection constituents are properly considered as types that can contextually type the
nameproperty here. The second one isn't eliminated soon enough - asT's type is still a work in progress. So thenameproperty gets typed based on__VERTER__TS__T & boolean(and that's basicallynever). And thus it widens to just astring- whereas previously it would only be typed based on__VERTER__TS__Tand given that has a string contraint... the type would not widen and it would stay as"foo".Please also remember that
Tcould be instantiated withnever, so - despiteThaving astringconstraint - that intersection member can't be easily eliminated.