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Union type is not compatible to second union type, even with full overlap. #30170
Description
TypeScript Version: 3.3.3
Search Terms:
Union types overlapping
Code
interface Blue {
color: 'blue'
}
interface Yellow {
color?: 'yellow',
}
function draw(val: Blue | Yellow) { }
function drawWithColor(currentColor: 'blue' | 'yellow' | undefined) {
// The following line throws the type error: Type '"blue"' is not assignable to type '"yellow"'.
return draw({ color: currentColor });
}Expected behavior:
No type errors exist, because there is an interface which would accept 'blue'.
Actual behavior:
An error is thrown:
Argument of type '{ color: "blue" | "yellow"; }' is not assignable to parameter of type 'Blue | Yellow'.
Type '{ color: "blue" | "yellow"; }' is not assignable to type 'Yellow'.
Types of property 'color' are incompatible.
Type '"blue" | "yellow"' is not assignable to type '"yellow"'.
Type '"blue"' is not assignable to type '"yellow"'.
Playground Link:
Link to the playground example
Activity
jack-williams commentedon Mar 1, 2019
It might be the case that a literal with type
{ x : A | B }is assignable to{ x: A } | { x: B }, but this does not hold when you have more properties. TypeScript wont special-case for single property objects.RyanCavanaugh commentedon Mar 1, 2019
#8289 / #12052 / #18421