rule proposal: disallow wider return type annotations

[gameroman]

Examples

Invalid

// explicit return type is wider than what the function actually returns

function a(): string | number {
  return "some string";
}

declare const v: 1 | 2;

function b(): 1 | 2 | 3 {
  return v;
}

Valid

// explicit return type matches actual return

function a(): string {
  return "some string";
}

declare const v: 1 | 2;

function b(): 1 | 2 {
  return v;
}

[mchevestrier]

FWIW, I added such a rule in my own ESLint plugin: wyrm/no-lax-return-type

See also wyrm/no-lax-array-type