Type parameter inference of function chained parameters and return values does not work.

[WearyMonkey]

I'm writing an API with life cycle methods that are called in a specific order. Each life cycle method can return a 'state' object that is passed to the next life cycle method. I would like the API to be strongly typed, and the type of the state parameters to be inferenced from the return type of the previous life cycle method.

TypeScript Version: 3.9.2, 4.0.0-dev.20200530

Search Terms:

chain inference inference parameter

Code

type Chain<R1, R2> = {
  a(): R1,
  b(a: R1): R2;
  c(b: R2): void;
}

function test<R1, R2>(foo: Chain<R1, R2>) {

}

test({
  a: () => 0,
  b: (a) => 'a',
  c: (b) => {
    const x: string = b; // Type 'unknown' is not assignable to type 'string'.(2322)
  }
});

Expected behavior:

No error.

Actual behavior:

Error 2322 on const x: string = b

Removing the a parameter removes the error and works as expected, e.g.

// ...same as above...

test({
  a: () => 0,
  b: () => 'a',
  c: (b) => {
    const x: string = b; // no error
  }
});

Explicitly providing the parameter type also works, e.g.

// ...same as above...

test({
  a: () => 0,
  b: (a: number) => 'a',
  c: (b) => {
    const x: string = b; // no error
  }
});

Playground Link:

https://www.typescriptlang.org/play?#code/C4TwDgpgBAwgFgQwJYDsA8AlAjAGihgJgD4oBeKAbwCgooEAKASgC59caoAjehV7F-AQDcHAMb1OfAgIBuAeyQATEQF8qVAGYBXFKOBI5KKMAgBnYJlyCi9DXLmt4ydNjyEijSurVUT5+tS0vFBMZCQADDgckiEInqQkAOQIiVG0oqwS8SSBtFCihuZQAB6s5gBOqADmZFwitGoqjCJAA
Related Issues:

[WearyMonkey]

Similar issue with a single function consuming its own return type:

type Single<R2> = {
  c(b: R2): R2;
}

function test<R2>(foo: Single<R2>) {

}

test({
  c: (b) => {
    const x: number = b; // Type 'unknown' is not assignable to type 'number'.(2322)
    return 0;
  }
});

https://www.typescriptlang.org/play?#code/C4TwDgpgBAyglgOwOYBsIB4BKAmAfFAXigG8AoKKAYwAoAjALihwEpGcBuUgX1NIDMArgkrA4AewRRgEAM7AseanzFjG8ZGgW5mJXj1LS51MhUqM6OgvhMUqEuVAAejBAIC2tCACdCUWp1soLwhgAS9JAAYAqB4uZk4gA

[noc7c9]

Facing the same issue.

I also found a possibly related issue: #38845

[RyanCavanaugh]

@ahejlsberg design limitation or did I miss something?

[ahejlsberg]

This is a design limitation. In the original example, the expressions provided for the b and c members are context sensitive. We therefore perform a first phase of type inference where we use wildcard values for those members. In this first phase we successfully infer number for R1, but we make no inferences for R2. We then use our inferences from the first phase to assign types to the contextually typed parameters, and since we have no inferences for R2, the b parameter is given type unknown.

In order to support this particular scenario we'd need additional inference phases, i.e. one per contextually sensitive property value, similar to what we do for multiple contextually sensitive parameters. Not clear that we want to venture there, and it would still be sensitive to the order in which object literal members are written. Ultimately there are limits to what we can do without full unification in type inference.