type T = (() => 1) & (() => 2) extends () => infer R ? R : unknown
- What is the reason that
Tis notnever(1 & 2)? - Does the type always come from the last function, or can it come from one of them?
type T = (() => 1) & (() => 2) extends () => infer R ? R : unknown
T is not never (1 & 2)?infer always selects the final overload when presented with an intersection (essentially showcasing different overloads), regardless of how it is defined. The last overload represents the ultimate member in the intersection or final definition in the code:
type One = (() => 1) & (() => 2);
type Two = { (): 1; (): 2; };
type MutuallyAssignable = One extends Two ? Two extends One ? true : false : false;
// ^? true
function three(): 1;
function three(): 2;
function three() { return 42 }
type Three = typeof three;
class C {
four(): 1;
four(): 2;
four() { return 42 }
}
type Four = C["four"];
type T<F> = F extends (...args: any[]) => infer R ? R : unknown;
type T01 = T<One>;
// ^? 2
type T02 = T<Two>;
// ^? 2
type T03 = T<Three>;
// ^? 2
type T04 = T<Four>;
// ^? 2
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