This repository attempts to repro what looks like a soundness issue in Flow, presumably related to a large number of circular dependencies.

The unsoundness is in src/lib/reducer/data/index.js:

/* @flow */
import ImplementationStatesIndex from './ImplementationStatesIndex';

function unsound(): string {
  return ImplementationStatesIndex.fromImplementationStates([]);
}

ret should normally be a ImplementationStatesIndex, but here, Flow lets us return this as a string. In fact, Flow's type inference appears to incorrectly conclude that the type of ret is any.

There is no warning prompting to add a type annotation to the definition of ImplementationStatesIndex (but doing so fixes the problem).

The issue goes away if we make changes practically anywhere. For example, remove PreparedImplementation and the PreparedImplementation import in src/lib/reducer/data/models/Implementation.js:

/* @flow */
export type ImplementationId = string;

export type Implementation = {
  id: ImplementationId,
};

Then, this yields the expected issue when running Flow:

Error ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ src/lib/reducer/data/index.js:6:10

Cannot return ret because ImplementationStatesIndex [1] is incompatible with string [2].

     src/lib/reducer/data/index.js
     3│
 [2] 4│ function unsound(): string {
     5│   const ret = ImplementationStatesIndex.fromImplementationStates([])
     6│   return ret;
     7│ }
     8│

     src/lib/reducer/data/ImplementationStatesIndex.js
 [1] 5│   static empty(): ImplementationStatesIndex {