Conditional type doesn't narrow primitive types

TypeScript Version: 3.1.0-dev.201xxxxx

Search Terms:

Code

type OnlyNumber<T extends number> = T

type ToNumber<T extends number | string> =
    T extends string ? undefined : OnlyNumber<T>

Expected behavior: No errors

Actual behavior: Error:

index.ts:37:47 - error TS2344: Type 'T' does not satisfy the constraint 'number'.
  Type 'string | number' is not assignable to type 'number'.
    Type 'string' is not assignable to type 'number'.

37     T extends string ? undefined : OnlyNumber<T>

Playground Link: https://www.typescriptlang.org/play/#src=type OnlyNumber<T extends number> %3D T type ToNumber<T extends number | string> %3D T extends string %3F undefined %3A OnlyNumber<T>

Related Issues:

[weswigham]

This is just a manifestation of how we don’t track negated constraints in the false branch of a conditional type.

[flushentitypacket]

I think I’ve run into the same issue but in a different context–just going to add this here to put a snippet in for anyone searching for answers for the same type of problem.

type OptionalFn = (() => string) | undefined
// Type 'OptionalFn' does not satisfy the constraint '(...args: any[]) => any'.
// Type 'undefined' is not assignable to type '(...args: any[]) => any'
type OptionalFnReturnType = OptionalFn extends undefined ? undefined : ReturnType<OptionalFn>

Can workaround by manually narrowing type:

type OptionalFn = (() => string) | undefined
type OptionalFnReturnType = OptionalFn extends undefined ? undefined : ReturnType<NonNullable<OptionalFn>>

(See NonNullable source in the Typescript docs)