Generator type inference error

[weswigham]

TypeScript Version: 2.1.6 and 2.2.0-dev.20170213

Code

tsconfig.json:

{
    "compilerOptions": {
        "module": "commonjs",
        "target": "es6",
        "noImplicitAny": true,
        "sourceMap": true
    }
}

test.ts:

export interface StrategicState {
    lastStrategyApplied?: string;
}

export function strategy<T extends StrategicState>(stratName: string, gen: (a: T) => IterableIterator<T | undefined>): (a: T) => IterableIterator<T | undefined> {
    return function*(state) {
        for (const next of gen(state)) {
            if (next) {
                next.lastStrategyApplied = stratName;
            }
            yield next;
        }
    }
}

export interface Strategy<T> {
    (a: T): IterableIterator<T | undefined>;
}

export interface State extends StrategicState {
    foo: number;
}

export const Nothing: Strategy<State> = strategy("Nothing", function*(state: State): IterableIterator<State | undefined> {
    return state;
});

Expected behavior:
Compiles without error.

Actual behavior:
On line 24, on the generator star: Generator implicitly has type 'IterableIterator<any>' because it does not yield any values. Consider supplying a return type.. This is unfixable without a terrible hack (add a line with the text if (!!false) yield state; to the top of the generator body), as there is already a return type specified. Actually, the return type (and argument type) should be inferred from the type annotation on the LHS of the equals sign and the strategy function signature (and the return type should not be required), but its not, though that's possibly a separate issue.

[yuit]

hi @weswigham !!! 🐱 This turned out to be an issue from how we aggregate return type from function body. We have a check for generator that only aggregate return type from yield expression...