Bug Report

🔎 Search Terms

• Return types
• Type inference

🕗 Version & Regression Information

• This is a bug
• This happens on all of the latest versions usable in the playground (4.1.3, 4.2.0-beta, even nightly)

⏯ Playground Link

Playground link with relevant code

💻 Code

declare function combined<T>(obj: {
  f1: (fixedParam: string) => T;
  f2: (inferredParam: T) => unknown;
}): unknown;

// Case 1
combined({
  f1: (fixedParam) => 1,
  f2: (inferredParam) => {}, // unknown
});

// Case 2
combined({
  f1: (fixedParam: any) => 1,
  f2: (inferredParam) => {}, // number
});

// Case 3
combined({
  f1: () => 1,
  f2: (inferredParam) => {}, // number
});

declare function separated<T>(
  obj1: { f1: (fixedParam: unknown) => T },
  obj2: { f2: (inferredParam: T) => unknown },
): unknown;

// Case 4
separated(
  { f1: (fixedParam) => 1 },
  { f2: (inferredParam) => {} }, // number
);

🙁 Actual behavior

There is a function with a type argument T. It requires an object as an argument that has two properties:

• A function that returns T
• A function that requires T as an argument.

If the first function requires any argument that must be inferred from the signature, TypeScript will fail to infer T (Case 1).

This behavior does not occur when:

• fixedParam is explicitly typed (Case 2) or not used (Case 3).
  or
• Two inner functions are in separate arguments of outer functions (Case 4).

🙂 Expected behavior

All inferredParam should be inferred as number. This feature is useful when struggling with handling complex data structure in React.