Missing errors in generic functions in context-sensitive arguments

Description

declare function fn<P>(config: {
  callback: (params: P) => (context: number, params: P) => number;
  unrelated?: (arg: string) => void;
}): (params: P) => number;

export const result = fn({
  callback: <T,>(params: T) => {
    return (a: boolean, b) => (a ? 1 : 0); // this should error somewhere, boolean !== number
  },
  unrelated: (_) => {},
});

Contributor guide

Tech stack

typescript

Domain

toolingdeveloper experience

Issue type

bug

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

4

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

over 1 week

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

fresh

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

clear

Prerequisites

TypeScript type systemTypeScript compiler internalsContextual typing

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

10

Research direction

The issue involves missing type errors in generic functions with context sensitive arguments. The playground link shows a case where `fn` expects a callback returning a function from `number` to `number`, but the provided function returns `boolean` without error. Comment suggests `anyFunctionType` leaked into `resolvedReturnType`. Start by examining the TypeScript compiler source in `src/compiler/checker.ts`, specifically the contextual typing and type inference logic for function signatures. Look for how `resolvedReturnType` is set and where errors for return type mismatches are reported. Also check `src/compiler/types.ts` for type relation checks.