Object created by rest operator keeps readonly status

[codepunkt]

When destructuring a readonly object with the rest operator, the new object that is created keeps the readonly flag on it's properties. Rest operator creates a new object the same way that spread operator does, so i assume it should not keep any flags.

For reference:
Issues #14058 and #19178 target the same problem with spread operator and have both been fixed.

TypeScript Version: 2.8.3

Search Terms: rest readonly, destructuring readonly

Code

type ObjType = {
  foo: string
  baz: string
  quux: string
}

const obj: Readonly<ObjType> = {
  foo: 'bar',
  baz: 'qux',
  quux: 'quuz',
}

const { foo, ...rest } = obj

delete rest.baz

Expected behavior:
Successful compilation

Actual behavior:
test.ts(15,8): error TS2704: The operand of a delete operator cannot be a read-only property.

[mhegazy]

I am convinced we fixed this already, but can not find a PR or a bug for it.. @sandersn did not we look at this issue before?

[sandersn]

No, I'm pretty sure it was for spread as @codepunkt said. I remember tweaking the code for spread, at least, and do not remember tweaking the code for rest.

Also, there is definitely no code to create synthetic properties in getRestType, unlike getSpreadType. It passes them straight on through:

for (const prop of getPropertiesOfType(source)) {
    const inNamesToRemove = names.has(prop.escapedName);
    const isPrivate = getDeclarationModifierFlagsFromSymbol(prop) & (ModifierFlags.Private | ModifierFlags.Protected);
    const isSetOnlyAccessor = prop.flags & SymbolFlags.SetAccessor && !(prop.flags & SymbolFlags.GetAccessor);
    if (!inNamesToRemove && !isPrivate && !isClassMethod(prop) && !isSetOnlyAccessor) {
	members.set(prop.escapedName, prop);
    }
}