generic function parameter infers boolean as true

[aleclarson]

TypeScript Version: 3.3.0-dev.201xxxxx

Search Terms: true boolean generic function extends default

Code

const fn = <T extends boolean>(flag: T | false = false) => flag
fn({} as boolean)

// The only workaround?
fn<boolean>({} as boolean)

Expected behavior:
Expected fn<boolean>(flag?: boolean) to be inferred

Actual behavior:
Instead got fn<true>(flag?: boolean) as the inferred type.

The issue stems from T | false, which is required for = false to work.

Playground Link: Click here

Related Issues: Possibly #28154

[MartinJohns]

<T extends boolean> means the only possible values for T are false and true. Your parameter is a union of T and false. Why are you even using a type parameter in this case? You get the same behavior by just dropping the type parameter and use boolean as the type.

[aleclarson]

Why are you even using a type parameter in this case?

@MartinJohns It's a minimal reproduction. The real use case is using T in a conditional type for another argument.

<T extends boolean> means the only possible values for T are false and true.

Or so you would think. You can do false & { __brand: void } (see here).