TypeScript: Conditional types are distributive over type parameters.

01_definition.md

Def. A conditional type with a checked naked type parameter T is called distributive conditional types.

Example:

// given U, X and Y
type DistributiveConditionalType<T> = T extends U ? X : Y;

Distributive conditional types are automatically distributed over union types during instantiation.

Example:

  DistributiveConditionalType<A | B | C>
= (A | B | C) extends U ? X : Y
= (A extends U ? X : Y) | (B extends U ? X : Y) | (C extends U ? X : Y)

02_question.md

Given

type DistributiveConditionalType<T1, T2> = T1 extends T2 ? true : false;

Then

  DistributiveConditionalType<never, 1> // why resolved as 👉 never?
= (never) extends (1) ? true : false    // why resolved as 👉 true?
= ??? // How does the 👉 distribution look like?

03_answer.md

✅ Solved distribution:

  DistributiveConditionalType<never, 1>
= never // never is the empty union, it distributes to nothing and therefore the type resolves to never

✅ Solved the expanded type:

  (never) extends (1) ? true : false
= true // no type extends anything by definition