Literal types and their unions should not be mutable

[KSXGitHub]

This code should not be valid:

let a: 'a' = 'a'
a += 'x' // 'typeof a' is still 'a' 

let b: 5 = 5
b += 7 // 'typeof b' is still 5

[jack-williams]

This seems ok to me, though I might be missing something. At the point in the expression b does have type 5, but after the expression the type of b is correctly widened.

Do you have an example where this leads to undesirable behaviour?

let a: 'a' = 'a';
a += 'x';
const x: 'a' = a; // string not assignable to 'a'

let b: 5 = 5
b += 7;
const y: 5 = b; // number is not assignable to 5

[KSXGitHub]

@jack-williams

This seems ok to me, though I might be missing something. At the point in the expression b does have type 5, but after the expression the type of b is correctly widened

I didn't know that, I didn't even expect that. But it is still undesirable behavior for me: When I explicitly assign a type to a variable, I expect the type to not change.

[jack-williams]

Here is a related issue: #18102

Which does point to inconsistent behaviour, for example:

let num1: 5 = 5;
num1 += 7; // ok
// num1 has type number;

let num2: 5 = 5;
num2 = num2 + 7; // error

I can't say if this is a bug, or by design to support certain JS idioms. Someone on the team will need to clarify that. I agree that the documentation is probably lacking on this either way though.

[ahejlsberg]

This is definitely not the desired behavior. In general x += 1 should checked the same way as x = x + 1, and that is not the case here.

[jack-williams]

Should the operator ++ be treated in the same way? That also widens the literal type.

[KSXGitHub]

@ahejlsberg You set milestone to 3.2 but I think this might be a breaking change

[collin5]

Opened a pull request (a few weeks back) to fix an issue related to this. #28344

[ahejlsberg]

I'm going to close this as a duplicate of #14745 and move the conversation to here.