Allow code to shadow definitions in the Foundation module

Author: iCharlesHu

lib/AST/NameLookup.cpp

@@ -660,39 +660,19 @@ static void recordShadowedDeclsAfterTypeMatch(
         }
       }
 
-      // 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
-      // latter rather than the former, which be the consequence of a more
-      // significant change to name shadowing in the future.
-      if (auto owningStruct1
-            = firstDecl->getDeclContext()->getSelfStructDecl()) {
-        if (auto owningStruct2
-              = secondDecl->getDeclContext()->getSelfStructDecl()) {
-          if (owningStruct1 == owningStruct2 &&
-              owningStruct1->getName().is("Data") &&
-              isa<FuncDecl>(firstDecl) && isa<FuncDecl>(secondDecl) &&
-              firstDecl->getName() == secondDecl->getName() &&
-              firstDecl->getBaseName().userFacingName() == "withUnsafeBytes") {
-            // If the second module is the Foundation module and the first
-            // is the NIOFoundationCompat module, the second is shadowed by the
-            // first.
-            if (firstDecl->getModuleContext()->getName()
-                  .is("NIOFoundationCompat") &&
-                secondDecl->getModuleContext()->getName().is("Foundation")) {
-              shadowed.insert(secondDecl);
-              continue;
-            }
-
-            // If it's the other way around, the first declaration is shadowed
-            // by the second.
-            if (secondDecl->getModuleContext()->getName()
-                  .is("NIOFoundationCompat") &&
-                firstDecl->getModuleContext()->getName().is("Foundation")) {
-              shadowed.insert(firstDecl);
-              break;
-            }
+      // Next, prefer any other module over the Foundation module
+      if (auto foundationModule = ctx.getLoadedModule(ctx.Id_Foundation)) {
+        if ((firstModule == foundationModule) !=
+            (secondModule == foundationModule)) {
+          // If second module is Foundation, then it is shadowed by first
+          if (secondModule == foundationModule) {
+            shadowed.insert(secondDecl);
+            continue;
           }
+
+          // Otherwise, the first declaration is shadowed by the second.
+          shadowed.insert(firstDecl);
+          break;
         }
       }
 

test/Frontend/Inputs/ShadowFoundation.swift

@@ -0,0 +1,3 @@
+public struct UUID {
+    public let uuidString = "FakeUUID"
+}

test/Frontend/foundation-module-shadowing.swift

@@ -0,0 +1,12 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module -emit-module-path %t/ShadowFoundation.swiftmodule -module-name ShadowFoundation %S/Inputs/ShadowFoundation.swift -disable-availability-checking
+// RUN: %target-typecheck-verify-swift -I %t -disable-availability-checking
+
+import Foundation
+import ShadowFoundation
+
+func f(_ uuid: UUID) -> Bool {
+    return uuid.uuidString == "FakeUUID"
+}
+
+func g(_ uuid: Foundation.UUID) { }