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Type inference issue with spread and generics #37934
Description
We notice the following issue described in this playground.
TL;DR;
The type inferred when merging two objects trough a generic function is always resolved to T & U which is inexact if T shares keys with U.
The correct inferred type of const mergeObjects = <A, B>(a: A, b: B) => ({ ...a, ...b });
should be
type Merge<T, U> = {
[k in Exclude<keyof T, keyof U>]: T[k];
} & {
[l in Exclude<keyof U, keyof T>]: U[l];
} & {
[l in keyof (T | U)]: T[l] | U[l]
};
<A, B>(a: A, b: B) => Merge<A, B>
As shown in the playground, the correct type is inferred when using the spread operator with concrete types.
Activity
RyanCavanaugh commentedon Apr 14, 2020
The proposed definition is much more complex yet still not fully accurate because it can't account for aliasing effects.
See discussion in #28234
typescript-bot commentedon Apr 16, 2020
This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.
flovilmart commentedon Apr 18, 2020
Thanks for pointing it out.