Type inference too narrow in Array.reduce with strictFunctionTypes on

[jakeboone02]

TypeScript Version: 3.4.1

Search terms: reduce strictFunctionTypes

Code

When the type of passing is explicitly set to boolean, this fails:

const passing: boolean = [1].reduce(pv => true, true);

Expected behavior:

Type inferred from true as the initialValue argument should be boolean.

Actual behavior:

Type 'boolean' is not assignable to type 'true'. <-- error on the return value of the reducer function. It's as if I wrote this:

const passing = [1].reduce(pv => true, true as true);

The workaround I'm using is this:

const passing: boolean = [1].reduce(pv => true, !!true);

This also works:

const passing: boolean = [1].reduce(pv => true, true as boolean);

Playground link:

Turn on strictFunctionTypes to see the error:

https://www.typescriptlang.org/play/#src=const%20passing%3A%20boolean%20%3D%20%5B1%5D.reduce(pv%20%3D%3E%20true%2C%20true)%3B

Related issues: none found

[ahejlsberg]

This issue has the same root cause as #30390, so I'm marking it as a duplicate.

[jakeboone02]

Thanks, @ahejlsberg. Should we just close this ticket, then?

[ahejlsberg]

Yes, I'll close it.