Change infer keyword behaviour to use it for type caching

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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

Beginne damit, die Beispiele für VERY_HEAVY_COMPUTATION, MakePair und Result aus dem Issue nachzustellen und das aktuelle Verhalten von A extends infer Cache ? ... : ... zu vergleichen. Bestimme, wie Typ-Caching funktionieren sollte, ohne die wiederholte bedingte Prüfung zu erfordern; abgeschlossen ist die Aufgabe, wenn die vorgeschlagene Result-Form typgeprüft wird und dabei den beabsichtigten inferierten Typ bewahrt.