Suggestion: stricter operators

Currently operators like "+" are defined such that they match their semantics in JS. The below are all allowed by the compiler and produce the shown values, even with --strictNullChecks on.

• 2 + 'a' => "2a"
• null + 'a' => "nulla" (!)
• 2 - null => 2

I propose letting users opt in (maybe via some --strictOperators) to strict operator behavior. Concretely I think this means:

• restrict +, and += to just number and string, e.g. for the former only declare

  function +(a: number, b: number): number;
  function +(a: string, b: string): string;
  

• restrict - and -= to just number

(any should continue to work as normal, of course.)

Relevant spec section:
https://github.com/Microsoft/TypeScript/blob/master/doc/spec.md#419-binary-operators

See also "Expression operators" in the strictNullTypes change: https://github.com/Microsoft/TypeScript/pull/7140
and in particular this rationale: https://github.com/Microsoft/TypeScript/pull/7140#issuecomment-186432250

This would fall under of "stricter" TypeScript, https://github.com/Microsoft/TypeScript/issues/274 .

doc/spec.md の Binary Operators セクションと、リンクされている strictNullTypes の議論から始め、その後 issue #274 の、より広範な stricter-TypeScript proposal を確認してください。+、+=、-、-= に対して提案されたオペランド型を opt-in オプションで強制しつつ、既存の動作を維持できれば、作業は完了です。