Generic inference from return type

[idoros]

TypeScript Version: 4.0.0-dev.20200715

Search Terms: generic inference

Code

interface Test<C> {
    title: string;
    test(context: C): unknown;
}
declare function suite<C>(setup: () => C, ...items: Test<C>[]): Test<C>;
declare function describe<PC, C extends PC>(setup: (context: PC) => C, ...items: Test<C>[]): Test<PC>;
declare function it<C>(title: string, test: (context: C) => unknown): Test<C>;

suite(
    () => ({
        a: 'param',
        b: 5
    }),
    it(``, ({a, b}) => {a.charAt(b)}), // <- generic works for first level
    it(``, () => {}),
    describe(
        ({a, b}) => ({a, b, c: true}),
        // it(``, ({a, c}) => {}) // <- uncomment to override generic ({a: any, c: any})
    )
);

Expected behavior:

Inference should work in the same way for describe as it works for suite.

Actual behavior:

Placing the it under the 2nd level triggers the inference in a different way. The difference between them is that in the describe signature the PC generic from the first level is used in the setup function - removing the context: PC from the setup function or just not using it in the actual setup will make both levels work the same.

Playground Link: Playground Link

Related Issues:

• Inconsistency in generic parameter inference

[RyanCavanaugh]

Looks like a job for #30134

[idoros]

@RyanCavanaugh , this still doesn't work in the latest/nightly version, is this something that will not get resolved, or just a duplicate?

[RyanCavanaugh]

@idoros it's unlikely this will be changed barring a full redesign of generic inference