Improve inference in the presence of context-sensitive expressions

[RyanCavanaugh]

Suggestion

🔍 Search Terms

contextually context-sensitive generic inference unused parameter method object literal

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

When generic inference is gathering candidates, context-sensitive expressions should still be considered if it's possible to do so without observing a type parameter from the call.

📃 Motivating Example

declare function callIt<T>(obj: {
    produce: (n: number) => T,
    consume: (x: T) => void
}): void;

// Works
callIt({
    produce: () => 0,
    consume: n => n.toFixed()
});

// Fails for no obvious reason
callIt({
    produce: _a => 0,
    consume: n => n.toFixed()
});

// Fails for no obvious reason
callIt({
    produce() {
        return 0;
    },
    consume: n => n.toFixed()
});

💻 Use Cases

Background: expressions can be "context-sensitive" for the purposes of inference -- this is a syntactic property of an expression that tells us whether or not an expression's type could be dependent on the inference of its contextual type. For _a => 0, this is true because the return expression might depend on _a (including in indirect ways). For produce() {, it's context-sensitive because (unlike an arrow function) it takes its parent object type for this, so could depend on the type parameter via a reference of this.

When it turns out that the type of the expression is not actually dependent on the inference of the type parameter (which would represent a true circularity, thus more understandably non-working), then this just looks busted for no obvious reason. This is a continuous source of surprise and annoyance. (TODO: link user reports)

Discussion

There are a few implementation strategies we could try, with varying trade-offs

• Proceed to do structural inference on context-sensitive expressions but mark the type parameters symbols as "off limits" with logic to back out. This is likely impractically invasive
• Perform a more robust check to determine "true" context-sensitivity -- for example, produce: _a => 0 should not be contextually sensitive because the relational target (n: number) => T does not use T in a covariant position. There are multiple ways we could do this
• Add an optional intermediate pass when inference collected no candidates when ignoring context-sensitive expressions where we go and collect from them anyway. This pass could safely fix parameters since they'd be going to unknown. This entire bullet point might be wrong.
• As a stopgap, consider function expressions to not be context-sensitive if all of their return expressions can be trivially seen to not depend on earlier lines of code (e.g. are literals, literals of literals, null, or undefined). This might fix a surprisingly large class of reports
• As a stopgap, consider an object literal method to not be context-sensitive if it doesn't reference this. I believe we already have code for this and it would fix another large class of surprises.

[ahejlsberg]

Fix now available in #48538.

[RyanCavanaugh]

Not all these cases work yet, so reopening

[ShayDavidson]

Might be related to my issue: #49951, which still happens in 4.7/4.8

[Ryuurock]

2023-10-13.15.09.40.mov

playground

[twiddler]

I reckon this causes this playground's issue, too? 🤔

[winstxnhdw]

I noticed that these issues always contain anonymous functions. I am getting the same issue using simple generic types. You can see it in this playground.

[martaver]

I've managed to conjure another example of this: https://tsplay.dev/mxpM7m

Referencing my SO issue and subsequent discussion for completeness.

[Andarist]

The inference source for R can only be discovered in the second inference pass today (the one that starts including context-sensitive nodes). The compiler never gets to it though.

The intermediate argument's type ({ x: {}; y: Fn<Foo>[]; }) gets rejected as it doesn't match the parameter type ({ x: (bar: Foo) => Base; y?: Fn<Base>[] | undefined; }) of the intermediate signature. Note that {} in x is the "anyFunctionType", that's an internal kind of any used as a placeholder in the first inference pass.

The problem is that the first pass will always use constraints as the fallback for the inferred types. Since - at that point in time - no candidate for R has been observed it gets instantiated as Base and the constraint of F gets instantiated as Fn<Base>. That isn't satisfied by Fn<Foo> that you pass there.

Do you even need F to be a type parameter? It doesn't appear in the return type of your Baz and that's usually a sign that it shouldn't be a type parameter.

You can work around this if you make an inference for R discoverable earlier using intersections:

interface Base {
  x: string;
}
interface Foo extends Base {
  y: number;
}

type Fn<T> = (t: T) => void;
declare const FooFn: Fn<Foo>;
declare const FooBase: Fn<Base>;
declare const FooExtra: Fn<Foo & { extra: string }>;

type Baz = <R extends Base, F extends Fn<R>>(arg: {
  x: (bar: Foo) => R;
  y?: (F & Fn<R>)[];
}) => R;

declare const baz: Baz;
const R1 = baz({ x: (ctx) => ctx });
const R2 = baz({ x: (ctx) => ctx, y: [FooFn] });
const R3 = baz({ x: (ctx) => ctx, y: [FooBase] });
const R$ = baz({ x: (ctx) => ctx, y: [FooExtra] }); // error (as expected, although the location of the error could be better)

[martaver]

Yeah, in a real world scenario, the type(s) of F would later be used to constrain other types.

The use of an intersection type's a great trick! Very handy to know...

One limitation, though, if I actually alter the return type of y then the compiler is back to expecting Base as the arg for it.

Here's an example where the return type is a union: https://tsplay.dev/N97gMW

I wonder if Fn<Base> is chosen because it's the narrowest common type between the return types, compiler trying to resolve Fn<Foo | Bar>... as opposed to Fn<Foo> | Fn<Bar>... forcing TS to distribute the Return type with R extends Base ? Fn<R> : never doesn't seem to change anything.

[Andarist]

Yeah, I don't have an answer to that 😉

An interesting experiment to run would be not ignoring the inferences that could be made from this anyFunctionType. If we'd infer any the signature applicability check could pass and the overall inference could improve in the second pass. I don't know when I'd find the time to play around with this though.