Unsafe type-incompatible assignments should not be allowed

TypeScript Version: 2.1.6

Code

interface StringOnly {
  val: string;
}

interface StringOrNull {
  val: string | null;
}

const obj: StringOnly = { val: "str" };

// should not be allowed
const nullable: StringOrNull = obj;

nullable.val = null;

// obj is now in a bad state

Expected behavior:
The sample should not compile, as it's not safe. Type casts/assertions should be required to override type incompatibility errors here. (Or, of course, cloning the object itself const nullable: StringOrNull = { ...str };).

For comparison, Flow does not allow the sample code.

Actual behavior:
The sample compiles and further accesses of str.val will likely result in exceptions.

EDIT: Removed all references to readonly, as discussion of that modifier is overshadowing the issue.

Commencez par compiler l’exemple TypeScript fourni et suivre le point d’entrée du type-checker qui accepte l’affectation de StringOnly à StringOrNull. Comparez les règles d’assignabilité qui en résultent avec la mutation non sûre indiquée, et tenez compte de la discussion existante avant de définir quel comportement et quelle couverture de régression seraient considérés comme terminés.