Generic type inference failed

[LiST-GIT]

🔎 Search Terms

Generics

🕗 Version & Regression Information

⏯ Playground Link

https://www.typescriptlang.org/play?#code/GYVwdgxgLglg9mABKSAeAUIriAqBGAGk2xwGZEBTADygrABMBnRAb0QG0BrCgTwC5EjKACcYYAOYBdAQAoICWjQH4AlIgC8APkQA3ODHqIAvkWy4ALBtYnEAeluIAwnAC2LurAmI4IKIigAFjDMjBTQ8EhwwIjy9BSILgCG3MwwfgDucMKciAAOwnC5FMIANjwAdOiaMvSJUIkCLMRYOMp4ANzNiADSyuQAZLhBjDg8Raj4iINkmp1Gak1G6OgoEDJNZq2sXWYARvsCeABMpKZm2ImXMguIwhRQIMJIpC-txmdYJl292+fY+7s5ApqFAFjs-jEEIw4CUKOUSnBxECwIooOVLolripOhDzvIwNDYfDEcjUeUAdjwX9AsFyhB6dccRCvrj6WsbizsF95u0gA

💻 Code

function func<
    T1,
    T3 extends { [key: string]: (context: T1) => void },
    T4 = {}, // Commenting out this section of code makes it work properly.
>(data: {
    T: T1;
    K: T3 & ThisType<T1 & T3>;
}) {
}

func({
    T: {
        bbb: 123,
        aaa() { return 333; },
    },
    K: {
        bbb(context) {
            console.log(context.aaa());
            console.log(context.bbb);
            this.ccc();
        },
        ccc() { },
    },
});

🙁 Actual behavior

display an error message:

'context' is of type 'unknown'.
'context' is of type 'unknown'.
'T4' is declared but its value is never read.
'data' is declared but its value is never read.

🙂 Expected behavior

The type of 'context' should already be determined; it shouldn't be 'unknown'.

Additional information about the issue

No response

[jfet97]

The following seems to work:

function func<
    T1,
    T3 extends { [key: string]: (context: T1) => void },
    T4 = {},
>(data: {
    T: T1;
    K: T3 & ThisType<T1 & T3>;
}) {
}

func({
    T: {
        bbb: 123,
        aaa: () => { return 333; }, // <-- here lies the difference: arrow instead of method
    },
    K: {
        bbb(context) {
            console.log(context.aaa());
            console.log(context.bbb);
            this.ccc();
        },
        ccc() { },
    },
});

Playground

[Andarist]

This likely belongs to this umbrella issue: #47599

[jfet97]

The problem is the following if inside instantiateContextualType:

// If no inferences have been made, and none of the type parameters for which we are inferring
// specify default types, nothing is gained from instantiating as type parameters would just be
// replaced with their constraints similar to the apparent type.
if (
  inferenceContext &&
  contextFlags! & ContextFlags.Signature &&
  some(inferenceContext.inferences, hasInferenceCandidatesOrDefault)
) {
    // For contextual signatures we incorporate all inferences made so far, e.g. from return
    // types as well as arguments to the left in a function call.
    return instantiateInstantiableTypes(contextualType, inferenceContext.nonFixingMapper);
 }

The condition results true at the wrong time, too eagerly as far as I understand it, therefore stuff gets instantiated with some default types.

If you change the condition in one of the two following ways you solve this specific issue, but you get failing baselines:

1. !some(inferenceContext.inferences, hasInferenceCandidatesOrDefault) seems somewhat more related with the above comment (?), but you get too many failing baselines
2. every(inferenceContext.inferences, hasInferenceCandidatesOrDefault) just a couple of failing baselines, but one of them is about contextual inference and, guess what, it works here but not there anymore

[mkantor]

This seems to have been fixed by TypeScript 6.0. The provided example no longer errors and context is now inferred as { bbb: number; aaa(): 333 }. I'm guessing that #62243 did the trick.