Iterable gets resolved incorrectly if value is passed directly and element has method

[InSyncWithFoo]

🔎 Search Terms

"iterable", "literal", "method"

🕗 Version & Regression Information

• This is the behavior in every version I tried:
• • 4.8.4
• • 4.9.5
• • 5.0.4
• • 5.1.6
• • 5.2.0-beta
• • 5.2.1-rc

💻 Code

// Attribution: https://stackoverflow.com/a/75687384
type NonStringIterable<T> = 
  T extends string ? never : T extends Iterable<any> ? T : never;

declare function doSomething<T>(value: NonStringIterable<T>): T;

const o = { foo() {} };

doSomething('value');
doSomething(['v']);
doSomething([o]);
doSomething([{ foo() {} }]);

🙁 Actual behavior

const o = { foo() {} };

doSomething('value');
//          ~~~~~~~
// Argument of type 'string' is not assignable to parameter of type 'never'. (2345)
// (correct)

doSomething(['v']); // Fine (correct)
doSomething([o]);   // Also fine (correct)

doSomething([{ foo() {} }]);
//          ~~~~~~~~~~~~~~
// Argument of type '{ foo(): void; }[]' is not assignable to parameter of type 'never'. (2345)
// (incorrect)

doSomething([o]) works correctly, yet directly passing the same object as o causes an error. This error happens regardless of the method being deeply nested or not. Interestingly, empty object ({}) and arrow function ({ foo: () => {} }) are fine. The same goes with a separately declared method.

🙂 Expected behavior

Only the first example should cause an error.

[Andarist]

My gut feeling is that this is related to the fact that this method is context-sensitive (since it's a method that has access to the this parameter). The containing object is blocked from being a source of inference based on that. Thus the constraint gets assigned to the inferred type here (since there are no other inference sources present) - that's unknown, and if we run this conditional type with unknown as T we get never as the result.

[andrewbranch]

This is one example of the kind of problem tracked at #47599.

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.