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Empty intersections as function return types are not treated as never #47833
Description
Bug Report
🔎 Search Terms
empty intersections, never, return type
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about types.
⏯ Playground Link
💻 Code
type A = { p: "A" };
type B = { p: "B" };
function f1(): never {
return;
}
function f2(): A & B {
return;
}
function g1(): never {}
function g2(): A & B {}🙁 Actual behavior
f1: Type 'undefined' is not assignable to type 'never'. (expected)
f2 w/ strictNullChecks: false: no error emitted
f2 w/ strictNullChecks: true: Type 'undefined' is not assignable to type 'never'. The intersection 'A & B' was reduced to 'never' because property 'p' has conflicting types in some constituents. (expected)
g1: A function returning 'never' cannot have a reachable end point. (expected)
g2: A function whose declared type is neither 'void' nor 'any' must return a value.
🙂 Expected behavior
f2 and g2 should generate the same errors as f1 and g1 respectively other than elaboration on empty intersection types.
Note: I was suggested to separate fix from #47751 and I'm ready to submit another PR if this is really a bug.
Activity
RyanCavanaugh commentedon Feb 10, 2022
I don't really see a defect per se here,
g2still has an error. We don't give it the special-cased error message aboutneverbut the error is effectively telling you the same thing, and why would someone write code like this on purpose?lowr commentedon Feb 10, 2022
Is it really telling you the same thing? If I see the current error message on
g2I'd try to return something until I notice the return type isnever. Worse, if I try addingreturn;at the end of the function just to try to please the compiler (which I shouldn't, of course!),g2becomesf2, which understrictNullChecks: falsewouldn't emit any error.I understand that the above code is contrived, but this may emerge in larger code bases with generics, possibly by unintentionally declaring a function's return type as an empty intersection. Quote from #36696 (comment):
Seeing no one else has ever reported this as a bug, I can argue that no one would actually confront something like this, but I'd say "fixing" this would make the compiler a bit more friendly to the users.
RyanCavanaugh commentedon Feb 10, 2022
It's correct to not error under
snc: falsebecause{ p: undefined }is a legal return valueRyanCavanaugh commentedon Feb 10, 2022
If it's straightforward to make this better we can take a PR, but if it's complicated or has other side effects I'd consider this "not a defect"
lowr commentedon Feb 10, 2022
I thought with #36696
A & Bis equivalentneverso all of the four functions aren't allowed to return anything even undersnc: false? Just double checking.