[SE-0491] Implement lookup and diagnostics for module selectors (MyMod::someName) #34556
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Woohoo! Thank you for reviving this, Becca! |
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@swift-ci test |
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@swift-ci test |
beccadax
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| // Disable module selector filtering--UnqualifiedLookupRequest should have | ||
| // done it. | ||
| LookupResultBuilder builder(result, dc, /*moduleSelector=*/Identifier(), | ||
| options); |
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@slavapestov This is the result of that review comment you made on the other PR.
tshortli
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Oct 23, 2025
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@swift-ci please test |
In this commit, this change affects certain diagnostics but doesn’t actually alter name lookup behavior yet.
Adds comments explaining why these conversions aren’t lossy.
Lookups like Builtin::Int64 were failing because BuiltinUnit rejected all unqualified lookups. Make it allow unqualified lookups with a module selector.
In code like the following:
```
protocol P { associatedtype A: Hashable }
protocol Q { associatedtype A: Comparable }
func fn<T: P & Q>(_: T) where T.A == Int { … }
```
`T.A` is actually the union of `P.A` and `Q.A`—it satisfies both associated types and has both of their constraints. This means it doesn’t actually make sense to apply a module selector to `A`—even if `P` and `Q` are in different modules, `T.A` always represents both of the declarations, not one or the other. We therefore now ban module selectors in this position, since they don’t actually jibe with the nature of a generic signature.
This justification technically doesn’t hold for *every* member type of a generic parameter—a member type can refer to a concrete typealias in a protocol extension, for instance—but in those situations, you can disambiguate (and add module selectors) by writing `P.A` or `Q.A` instead of `T.A`, so we’re not really worried about this limitation.
Approved by SE-0491.
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This pull request implements the lookup behavior, including tailored diagnostics, for "module selectors", which allow references to declarations to be unambiguously prefixed with a module they can be found through:
Since SE-0491 has been approved and this PR is sufficient to make them functional, it also changes them from an experimental feature to an ordinary on-by-default language feature.
Parsing for this feature has already landed in #84362. Conditional emission into module interface files will land in a future PR.
This work was previously in PR #28834, which was closed during the master-to-main transition.
Fixes rdar://problem/19481048.