Merge pull request #66998 from DougGregor/observation-shadowing-5.9

Author: DougGregor

include/swift/AST/KnownIdentifiers.def

@@ -125,6 +125,8 @@ IDENTIFIER(objectAtIndexedSubscript)
 IDENTIFIER(objectForKeyedSubscript)
 IDENTIFIER(ObjectiveC)
 IDENTIFIER_(ObjectiveCType)
+IDENTIFIER(Observation)
+IDENTIFIER_WITH_NAME(Observation_, "_Observation")
 IDENTIFIER(oldValue)
 IDENTIFIER(Optional)
 IDENTIFIER_(OptionalNilComparisonType)

lib/AST/NameLookup.cpp

@@ -680,6 +680,25 @@ static void recordShadowedDeclsAfterTypeMatch(
         }
       }
 
+      // Next, prefer any other module over the (_)Observation module.
+      auto obsModule = ctx.getLoadedModule(ctx.Id_Observation);
+      if (!obsModule)
+        obsModule = ctx.getLoadedModule(ctx.Id_Observation_);
+      if (obsModule) {
+        if ((firstModule == obsModule) != (secondModule == obsModule)) {
+          // If second module is (_)Observation, then it is shadowed by
+          // first.
+          if (secondModule == obsModule) {
+            shadowed.insert(secondDecl);
+            continue;
+          }
+
+          // Otherwise, the first declaration is shadowed by the second.
+          shadowed.insert(firstDecl);
+          break;
+        }
+      }
+
       // The Foundation overlay introduced Data.withUnsafeBytes, which is
       // treated as being ambiguous with SwiftNIO's Data.withUnsafeBytes
       // extension. Apply a special-case name shadowing rule to use the

test/NameLookup/Inputs/HasObservable.swift

@@ -0,0 +1,3 @@
+public enum Observable<Value> {
+  case just(Value)
+}

test/NameLookup/Inputs/ReexportsObservation.swift

@@ -0,0 +1,2 @@
+@_exported import _Observation
+

test/NameLookup/observable_shadowing.swift

@@ -0,0 +1,19 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module -o %t %S/Inputs/HasObservable.swift
+// RUN: %target-swift-frontend -emit-module -o %t %S/Inputs/ReexportsObservation.swift
+// RUN: %target-swift-frontend -typecheck %s -I %t -verify
+// RUN: %target-swift-frontend -typecheck %s -I %t -verify -DVIA_REEXPORT
+
+// REQUIRES: observation
+
+import HasObservable
+
+#if VIA_REEXPORT
+import ReexportsObservation
+#else
+import _Observation
+#endif
+
+func foo() -> Observable<Int> {
+  return .just(42)
+}