Uninhabited != NoReturn #4365
Uninhabited != NoReturn #4365
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Previously, isNever would return true for an equivalence class of
uninhabited enums. The rest of the compiler, however, is using this in
places that actually expect just the Never type. This means that code
like this would compile properly
enum Uninhabited {}
func do() -> Uninhabited { /* No body here */ }
and we wouldn’t diagnose the missing return.
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@swift-ci please smoke test. |
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Why do we want this? |
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There's a clear difference between |
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There's not intended to be. We need to diagnose "falling off the end" in either case. |
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Right. To clarify, we want any uninhabited type to behave like Never, and mark a function as not returning. Right now the isNever check is inaccurate because it doesn't realize that a strict with an uninhabited field is also uninhabited, but that didn't seem worth fixing. |
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@jrose-apple By definition we cannot fall off the end if the return type is Never, because the return is not reachable. |
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When dealing with user-defined types it's more likely that they meant to write up the cases for the enum later and would expect a diagnostic about the function not returning than they decided to exploit enum X {}
func explode() -> X {
return explode()
} |
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@swift-ci please smoke test Linux platform. Smoke test link |
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If defer to @jckarter regarding this. It was his original suggestion to treat all empty enums as a 'Never' type. |
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Actually, the fact that this doesn't produce a diagnostic is the real bug: It happens for both 'E' and 'Never'. Mind if I take this? |
| @@ -9,6 +9,11 @@ func singleBlock2() -> Int { | |||
| y += 1 | |||
| } // expected-error {{missing return in a function expected to return 'Int'}} | |||
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| enum NoCasesButNotNever {} | |||
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| func diagnoseNoCaseEnumMissingReturn() -> NoCasesButNotNever { | |||
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Note that this is meant to be an error even with a 'Never' return. a Never-returning function should end with a call to some other Never-returning function, or the return should be unreachable.
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That it happens for |
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@CodaFi That's the thing, we should get the diagnostic for Never too. Remember, this was invalid before: A @NoReturn function had to either end with a call to some other @NoReturn function, or end with an infinite loop, essentially. You weren't allowed to fall through the end. A return type of Never should behave exactly like a @NoReturn function did in Swift 2. It looks like something is wrong in the SIL unreachable diagnostics pass wrt the Never type. |
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Sounds good. |
What's in this pull request?
Fix a case where we would fail to diagnose a missing return if an enum decl as the result of a function had no cases.
@slavapestov can you review?
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