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.