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Literal types and their unions should not be mutable #28077
Description
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{{ message }}
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
Activity
[-]Literal types and and their unions should not be mutable[/-][+]Literal types and their unions should not be mutable[/+]jack-williams commentedon Oct 23, 2018
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?
KSXGitHub commentedon Oct 23, 2018
Jack Williams (@jack-williams)
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 commentedon Oct 23, 2018
Here is a related issue: #18102
Which does point to inconsistent behaviour, for example:
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 commentedon Oct 23, 2018
This is definitely not the desired behavior. In general
x += 1should checked the same way asx = x + 1, and that is not the case here.jack-williams commentedon Oct 23, 2018
Should the operator
++be treated in the same way? That also widens the literal type.KSXGitHub commentedon Oct 23, 2018
Anders Hejlsberg (@ahejlsberg) You set milestone to 3.2 but I think this might be a breaking change
collin5 commentedon Nov 21, 2018
Opened a pull request (a few weeks back) to fix an issue related to this. #28344
ahejlsberg commentedon Feb 1, 2019
I'm going to close this as a duplicate of #14745 and move the conversation to here.