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keigoi/nested_bogus.ts

Created October 15, 2020 23:14
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TypeScript strange behaviour in nested conditional types
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nested_bogus.ts
type GetFoo<T> = [T] extends [{foo:infer R}] ? R : never;
type Foo = GetFoo<{foo:"A"}> // "A"
type GetFoo0<T> = [T] extends [{foo:string}] ? [T] extends [{foo:infer R}] ? R : never : never;
type Bogus = GetFoo0<{foo:"A"}> // unknown ???
// to remedy this, use indexed type T["foo"]
type GetFoo1<T> = [T] extends [{foo:string}] ? [T["foo"]] extends [infer R] ? R : never : never;
type Foo1 = GetFoo1<{foo:"A"}> // "A"
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