Unambiguous disjoint union treated as ambiguous

[skeggse]

Flow version: v0.92.1

Expected behavior

This code should produce no errors:

type StrangeOptional = {allow: string[]} | {allow: null};

// This should be easy for Flow to understand - it's clearly case [1] and not case [2].
({allow: ['com']}: StrangeOptional);

Actual behavior

5: ({allow: ['com']}: StrangeOptional);
    ^ Could not decide which case to select. Since case 1 [1] may work but if it doesn't case 2 [2] looks promising too. To fix add a type annotation to array literal [3].
References:
3: type EitherOptional = {allow: string[]} | {allow: null};
                         ^ [1]
3: type EitherOptional = {allow: string[]} | {allow: null};
                                             ^ [2]
5: ({allow: ['com']}: EitherOptional);
            ^ [3]

Workaround

// Explicitly telling Flow that this is a string array seems to fix the error.
({allow: (['com']: string[])}: StrangeOptional);

• Link to Try-Flow or Github repo.

[SamChou19815]

This now works