Closed
Description
TypeScript Version: 2.0.0 beta
Functions typed to accept union typed params incorrectly typecheck against functions whose params only match a subset of the union. Example:
interface Callback {
(value: string | number): void;
}
// expect this to work
const many: Callback = (value: string | number | boolean) => {
// ...
}
// expect a type error here
const one: Callback = (value: string) => {
// ...
}
many(1); // should work, does work
one(1); // shouldn't work, does work *
many("hello"); // should work, does work
one("hello"); // should work, does work
many(true); // shouldn't work, doesn't work
one(true); // shouldn't work, doesn't workExpected
one throws a type error, since string | number is not assignable to string.
Actual
one compiles fine.
This comes up in real code when declaring a callback whose params may not be defined, e.g.
interface SomeLib {
onChange: (callback: (newValue: string | null) => void) => void;
}
const foo: SomeLib = ...;
foo.onChange((newValue: string) => {
// oops, I forgot newValue may not be defined, but this compiles
});