Deleting from readonly dictionary does not give a type error

[paulkoerbitz]

TypeScript Version: 2.0.0, 2.0.3, nightly (2.1.0-dev.20161010)

Code

// A *self-contained* demonstration of the problem follows...
'use strict';

interface X { readonly [key: string]: string }

const x: X = Object.freeze({
    "a": "A",
    "b": "B",
    "c": "C"
});

// Error: TS2450: Left-hand side of assignment expression cannot be a constant or a read-only property.
x["a"] = "B";

// No error - but runtime exception
// TypeError: Cannot assign to read only property 'a' of object '#<Object>'
delete x["a"];

Expected behavior:
delete x["a"]; should give a compile-time error (as it predictably leads to a runtime error later on).

Actual behavior:
No compile-time error is given.

Design Limitation (?)
I understand that this may be a design limitation as in #11180, in this case, this is another vote for the --strictReadonlyChecks flag discussed in #11180.

[RyanCavanaugh]

Seems reasonable to disallow delete operator on any readonly property or index signature

[RyanCavanaugh]

Approved for PRs. Shouldn't be too complex.

[mhegazy]

thanks @HerringtonDarkholme !