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.

Start by compiling the provided TypeScript sample and tracing the type-checker entry point that accepts the assignment from StringOnly to StringOrNull. Compare the resulting assignability rules with the stated unsafe mutation, and consider existing discussion before defining what behavior and regression coverage would count as done.