Change infer keyword behaviour to use it for type caching

Search Terms

infer type cache

Suggestion

I saw one proposal somewhere, but I can't find it now. There people asked to add possibility of using type level variables inside type level computations. I realized now that simple constructions with infer keyword may be used for this after a small behaviour change:

type VERY_HEAVY_COMPUTATION<T> = T extends number ? 42 : []

type MakePair<P extends number, R extends number> = [P, R]

type Result = VERY_HEAVY_COMPUTATION<0> extends infer Cached
  // Error: Type 'Cached' does not satisfy the constraint 'number'.ts(2344)
  ? MakePair<Cached, Cached> : never

Currently in this case VERY_HEAVY_COMPUTATION<0> doesn't share its value with Cached automatically, so we need compute the value twice.

type Result = VERY_HEAVY_COMPUTATION<0> extends infer Cached
  ? Cached extends VERY_HEAVY_COMPUTATION<0>
    ? MakePair<Cached, Cached> : never
  : never

I suggest, in the case of such constuctions A extends infer Cache ? ... : ..., automatically share the type of A with Cache

Start by reproducing the issue's VERY_HEAVY_COMPUTATION, MakePair, and Result examples and compare the current behavior of A extends infer Cache ? ... : .... Determine how type caching should work without requiring the repeated conditional check; done means the proposed Result form type-checks while preserving the intended inferred type.