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Declaration order dependence when inferring to a union #45603
Description
Bug Report
🔎 Search Terms
complex types generic inference
🕗 Version & Regression Information
TS Version: ^4.2
- This is a crash
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about incorrect types function parameters
⏯ Playground Link
Playground link with relevant code
💻 Code
interface Action<TName extends string,TPayload> {
name: TName,
payload: TPayload
}
const actionA = { payload: 'any-string' } as Action<'ACTION_A', string>
const actionB = { payload: true } as Action<'ACTION_B', boolean>
function call<TName extends string,TPayload>(
action: Action<TName,TPayload>,
fn: (action: Action<TName,TPayload>)=> any,
) {
fn(action)
}
const printFn = (action: typeof actionA | typeof actionB)=> console.log(action)
// It works
call(actionA, printFn)
// It crashes
call(actionB, printFn)
//workaround
call<'ACTION_B', boolean>(actionB, printFn)
Activity
MartinJohns commentedon Aug 27, 2021
There's some weird wonkyness in the Playground. Depending on whether you hover over the first or second
callfunction call you get a different tooltip for the first call:andrewbranch commentedon Aug 27, 2021
Perhaps a better workaround: https://www.typescriptlang.org/play?ts=4.5.0-dev.20210826#code/JYOwLgpgTgZghgYwgAgIILMA9iAPAFQDk4BbFCAD0hABMBnZOsKUAcwBp8AFOATwBsscGgD5kAbwBQyGchCkIALmREF7abIAOfQcOXcdQmpIC+kyQhxNkiTDgBCyALwTk2gUeXMArihM2GdDs8AHJUAGF8AEkAeUIAfXsQ9mQAIywsfgg4EBELKzAbDGwQVGdXd10aZRCc3gBaJhYQVhDkfzhA4pxcMMjYhNRkxmY2PMkYbxBukGQEOH5+AiCS5EpqejQZ3Ca2FKmAaxAsAHdckQAKDVsS-RWcdRkYEGULm5w7mYBKJzE69S+Eg0zze31M5ksIGsmmaYAAYrMXKDgl5eJoIFgYEVgmUAD7IMBojFY94geyA35zKyZCAAOkErGRJS+EIW-CZOFQKRhoHhIBZ80WHLJ3NhCJZQA
[-]Cannot infer more than one generic type in a function that receives an object and another function as parameters[/-][+]Declaration order dependence when inferring to a union[/+]andrewbranch commentedon Aug 27, 2021
The thing that makes this particularly bad is that which call errors is dependent on whether
actionAoractionBis declared first (and therefore the internal ordering of the uniontypeof actionA | typeof actionB).afonsof commentedon Aug 31, 2021
That looks great! However I simplified the problem a bit to a better understanding. So this workaround does not work in my case.
It has two interfaces sharing the same generics. Specifically
PayloadActionCreatorandPayloadAction. These are part oftypesafe-actionslibTake a look at this new sample:
https://www.typescriptlang.org/play?ts=4.5.0-dev.20210826#code/JYOwLgpgTgZghgYwgAgIILMA9iAPAFQDk4BbFCAD0hABMBnZOsKUAcwBp8AFOATwBsscGgD5kAbwBQyGchCkIALmREF7abIAOfQcOXcdQmpIC+kyaEixEKdJhwB1LFADW0AsTLJK1eo2ZsnDwCRmJSsnIK+p4Q6hHaIXoqwbrGZpIIOEzIiPYgAELIALwSyAmpyswArigmOQx22CBOru4A5KgAwvgAkgDyhAD6+W3syABGWFj8EHAgIhlZYDkYTajFpeVGym1zvAC0TCwgrG3IdXANq47OblC4Hd39Q6ij-sesC5IwVSDXIMgEHB+PwCPheJpyFQILQGEdAslDMIRAAKDS5JrKRo4MEQ2KIxKiOLIGAgZQojE4LH-Fp3XGQoJI0QASiKYj26mZEg0pIp-2ZpnMmRA2U0xzAADEASU+XlKnisDAVnl1gAfZBgBVKykFLlswFZaYQAB0glYsqaAoywP4FpwqDGYssUoFQJBdoKjvFLskQA
ahejlsberg commentedon Oct 15, 2021
Simpler repro:
The issue here is that we make two contra-variant inferences for
T(namely'a'and'b') and then pick one of the two based on the sort order of type identities--which so happens to be dictated by declaration order. We have logic to prefer a co-variant inference if it is a subtype of the contra-variant inference, but that happens after we've (randomly) picked one of the contra-variant inferences.What we really should do is check if the co-variant inference is a subtype of any of the contra-variant inferences, and, if so, go with the co-variant inference. The random picking should really be a last resort since we know it's going to lead to an error downstream (it's still preferable to intersecting the contra-variant inferences because that'll produce a
neverwhen the inferences are disjoint).