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3.4.0-rc - behaviour changed from 3.3 (regression?) #30489
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[-]3.4 - behaviour changed from 3.3 (regression?)[/-][+]3.4.0-rc - behaviour changed from 3.3 (regression?)[/+]on Mar 19, 2019 This is caused by #29437 I think. Not sure I would class this as a regression: is it causing negative behaviour?
At least for me :) , I use it like this
type ExtendIfI1<T> = T extends I1<any> ? T & Something : T
I can't use
T extends I1<infer X> ? I1<X> & Something : Tsince if I1 had more stuff than I1 it would be lost, and having to write infer on the left for it to work was hard to find outI think it boils down to the generic being inside the template part, this is
type A<T> = T extends any ? T : never type B = A<number> // this is any & number, which is number type O<T> = T extends I<any> ? T : never type B = O<I<number>> // could this be changed so it generates I<any & number>, which is I<number> // rather than I<any> & I<number> ?
This is the intended behavior to improve inference elsewhere by "collapsing" conditional types into plain types when we detect that's possible.
One option is to use
type O<T> = T extends I<unknown> ? T : never
This will produce the type
I<unknown> & I<number>which is indistinguishable fromI<number>but also a little ugly.You can also make the conditional non-distributive by writing
type O<T> = [T] extends [I<unknown>] ? T : never
This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.
While the original behavior was intended, its interaction with
anyis undesirable - to reduce breakage and make better output types, in #30592 we introduce supertype reduction on substitution type instantiation, which should eliminate needlessany'd up intersection members in most cases.Reacted by Javier Gonzalez
I found a case where types yielded differ between 3.3 and 3.4, but I'm not sure if it is a regression or it is intended