Exclude
Challenge
Implement the built-in Exclude<T, U>. Exclude from T those types that are
assignable to U. For example:
type T0 = Exclude<"a" | "b" | "c", "a">; // expected "b" | "c"
type T1 = Exclude<"a" | "b" | "c", "a" | "b">; // expected "c"
Solution
The important detail here is a knowledge that conditional types in TypeScript are distributive.
So that when you are writing the construct T extends U where T is the union,
actually what is happening is TypeScript iterates over the union T and applies
the condition to each element.
Therefore, the solution is pretty straightforward. We check that T can be
assigned to U and if so; we skip it:
type MyExclude<T, U> = T extends U ? never : T;
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