The return value of function type is not checked when function may return any

TypeScript Version: 4.2.0-dev.20201124

Search Terms:

function, type check, any

Code

const f1: () => number = () => {
  if (window) {
    return {} as any;
  }
  return "a"; // Pass
};

// Expected behavior
const f2: () => number = () => {
  return "a"; // Error: Type '() => string' is not assignable to type '() => number'
};

// Expected behavior
function f3(): number {
  if (window) {
    return {} as any;
  }
  return "a"; // Error: Type 'string' is not assignable to type 'number'.
};

Expected behavior:

f1 causes an error because it returns a string.

Actual behavior:

f1 does not cause an error.

Playground Link:

https://www.typescriptlang.org/play?ts=4.2.0-dev.20201124#code/MYewdgzgLgBAZgRgFwwBQEoYF4B8MwCuAtgEYCmATtmprjAN4BQMMAlnGgO6tgAmInTExYsKZKAQpgGAXxgBDCArABPANzMYMzWIlSYAInkG1MAPRmYABUURGMjYwswAogA8ADmWBQyvGOQAFvIAbqwgFIygkLBwAEwoGNh4hKSU1El0wjC6ktJGJuaWLhQUESgAKipeMADkmXjQFDwA5rVsSmAgsLasLWDyJAA2ZDBQIGPVo-W0KcTkFLX2js7uXj5+AWTBYRGMcARgPuHScADMGCipCwya7Fw8-IK3IjniebIKSvKqGizaone+gKplWpXKMCqNVqTVa7VYnW6XwgfQGw1G40m0OulFqADplowgA

Start with the linked TypeScript Playground reproduction and compare the checks for f1, f2, and f3, focusing on return-type validation when one branch returns any. Trace the compiler's function return checking until the string return is diagnosed; done when f1 reports the expected type error without changing the valid cases.