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 ``` js 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 .