Object properties are inferred in the wrong order: should infer properties with `NoInfer<T>` AFTER properties with `T`

[denis-migdal]

🔎 Search Terms

NoInfer
brittle inference
object properties

Related issues:

• Parameter type inference/capture depends on the order of declaration of fields in an object. #56297 : inference depends on the order of the properties. This explains the inconsistency in my issue.

🕗 Version & Regression Information

Tested in 6.0.2 and nightly.

⏯ Playground Link

Playground Link

💻 Code

See playground for the full example.

type fooArgs<T> = {
     a: (_: A) => X<T>,
     b: Y<NoInfer<T>>
}

function foo<T>(args: fooArgs<T>) {}

foo({
     a: (_) => ...,
     b: ... // inference of the value given to b is VERY brittle.
});

🙁 Actual behavior

Sometimes, b is inferred before a.

Then, as a is the one used to infer the generic parameter T, the value given to b doesn't have the correct type (because inferred before a).
This is likely due to the fact that the value given to a is a callback with a parameter we didn't specify the type ((_) => ...). Therefore, TS has first to look at fooArgs<T> to infer the callback's parameter type (i.e. the type of _).

Due to that, b is sometime inferred before a has been properly inferred. This behavior depends on the order of the object properties (cf related issue). It is also influenced by other defined properties, and how they are defined. Making the inference quite chaotic.

🙂 Expected behavior

TS should infer NoInfer<T> properties AFTER T properties, in order to prevent such issues.

Additional information about the issue

The fact that the order of the inference changes is quite troublesome, hiding the issue in some cases.

Could be nice to have a kind of tool/flag to detect such kind of potential issues, and to help debugging.

EDIT: A possible workaround:

type fooArgs<T> = {
     b: Y<NoInfer<T>>
}

function foo<T>(a: (_: A) => X<T>, args: NoInfer<fooArgs<T>>) {}

foo(
     a: (_) => ..., {
         b: ...
     });