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Empty array in union with tuple doesn't infer undefined on indexed access #47531
Description
Bug Report
declare const tuple: never[] | [string];
tuple[0].length // no compile-time error :(
Playground
Note that noUncheckedIndexedAccess is enabled - but doesn't make any difference in this case.
The reason it is a problem is that [] is inferred as never[] in many scenarios, thus allowing type-unsafe code. Example:
const data = ['A'] as const;
// inferred as `never[] | ['A']
const a = Math.random() > 0.5 ? [] : data;
a[0].length; // can be undefined, but no compiler error
🔎 Search Terms
Tuple, union, empty array, infer, noUncheckedIndexedAccess
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about
never,tuple,array
⏯ Playground Link
Playground link with relevant code
💻 Code
declare const tuple: never[] | [string];
tuple[0].length // no compile-time error :(🙁 Actual behavior
Compiles well, breaks at runtime
🙂 Expected behavior
Compile-time error that tuple[0] is possibly undefined
Activity
[-]Empty tuple in union doesn't infer undefined on indexed access[/-][+]Empty array in union doesn't infer undefined on indexed access[/+]amakhrov commentedon Jan 20, 2022
This would not be a problem if an empty array was inferred as
[](empty tuple) instead ofnever[][-]Empty array in union doesn't infer undefined on indexed access[/-][+]Empty array in union with tuple doesn't infer undefined on indexed access[/+]fatcerberus commentedon Jan 20, 2022
Workaround:
amakhrov commentedon Jan 20, 2022
Bruce Pascoe (@fatcerberus) yup.
From the practical standpoint, if I know I need this workaround here and now, it means I anticipate possible type unsafety and have double checked my code already. However, if I'm unaware I'm doing something wrong, it would also not occur to me to do
as const.So my point is that this should be as automated (inferred) as possible.
RyanCavanaugh commentedon Jan 20, 2022
This is pretty messed up; we shouldn't be presenting a
never[]under normal circumstances.fatcerberus commentedon Jan 22, 2022
Ryan Cavanaugh (@RyanCavanaugh) A surefire way to get a
never[]is to simply return[]from a function:jcalz commentedon Aug 28, 2022
#50474 was marked as a duplicate of this, but if so, then the problem doesn't seem to have much to do with
never[], and has more to do with propagation of theundefined(when--noUncheckedIndexedAccessis enabled) when reading properties from unions where some union members have index signatures and some do not. Like, what do we make of this behavior:Playground link
Is this a bug? If so, does it belong here in #47531 or somewhere else?