Type not captured in generic functions when using a configuration object

[ricokahler]

TypeScript Version: 2.3.2

The type isn't captured when using generics inside of a configuration object in some cases.

In particular, the function functionAThatTakesAnObject should capture the type of aNumber so that the type of shouldBeInferedToBeANumber should be number but instead it's given type {}

Code

function functionA<T>(funcThatReturnsT: (something: any) => T, funcThatTakeT: (t: T) => any) {
    let something = undefined as any;
    let hasTypeT = funcThatReturnsT(something);
    return funcThatTakeT(hasTypeT);
}

let aNumber = 5;

// works
functionA(() => aNumber, shouldBeInferedToBeANumber => shouldBeInferedToBeANumber.toExponential());
// still works
functionA(_ => aNumber, shouldBeInferedToBeANumber => shouldBeInferedToBeANumber.toExponential());

/**
 * this is the same as `functionA` but it takes a configuraton object instead
 */
function functionAThatTakesAnObject<T>(opts: {
    funcThatReturnsT: (something: any) => T,
    funcThatTakeT: (t: T) => any
}) {
    let something = undefined as any;
    let hasTypeT = opts.funcThatReturnsT(something);
    return opts.funcThatTakeT(hasTypeT);
}

// works
functionAThatTakesAnObject({
    funcThatReturnsT: () => aNumber, // if there is no parameter, it works
    funcThatTakeT: shouldBeInferedToBeANumber => shouldBeInferedToBeANumber.toExponential()
});

// DOES NOT WORK
functionAThatTakesAnObject({
    funcThatReturnsT: _ => aNumber, // if there is a parameter, it breaks
    funcThatTakeT: shouldBeInferedToBeANumber => shouldBeInferedToBeANumber.toExponential()
});

Expected behavior:

shouldBeInferedToBeANumber should be inferred to be of type `number

Actual behavior:

shouldBeInferedToBeANumber is given type {}

[ricokahler]

I asked about this on stackoverflow and someone discovered that when r...

// this works!
functionAThatTakesAnObject({
    funcThatReturnsT: (r: any) => aNumber, // if there is a parameter, it breaks
    funcThatTakeT: shouldBeInferedToBeANumber => shouldBeInferedToBeANumber.toExponential()
});

...in the call functionAThatTakesAnObject has it's type explicitly stated, then there is no error. I find this strange because r already had the type of any but it works when I re-assert that is has type of any.

---

Here is a typescript playground showing the error.

Thanks for any help!

[RyanCavanaugh]

Using context-sensitive functions' return types for inference requires a unification-based algorithm; see #30134