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keyof does not work with extends in generic type argument. #16740
Description
TypeScript Version: 2.3.4
Code
function shallowCopy<T, U extends T>(s: T, d: U) {
Object.keys(s).forEach((k: keyof T) => d[k] = s[k]);
}Expected behavior:
I would expect this function to compile, since U is a sub class of T and U's keys are presumably treated as superset of T's keys. Each key of T should be a key of U as well.
Actual behavior:
TypeScript reports the following error: "Type 'T[keyof T]' is not assignable to type 'U[keyof T]'."
Activity
ahejlsberg commentedon Jun 27, 2017
As Andy (Andrewkraft) (@Andy-MS) is pointing out, this is working as intended. You are right that each key of
Tis a key ofUas well, but aU[K]may be a subtype of the correspondingT[K]for the sameK, andT[K]is therefore not assignable toU[K].mhegazy commentedon Aug 17, 2017
Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.