From f4dedc32b5d16a54a3d3285fe2eea8724c28dc6c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alexis=20Laferri=C3=A8re?= <alaf@apple.com>
Date: Fri, 4 Apr 2025 13:39:27 -0700
Subject: [PATCH] Serialization: Bring back shadowing but only after the
 filtering

Followup fix to #80009. We can still get ambiguities from colliding
decls across modules with the deserialization filtering. Bring back
calling the general lookup shadowing after the filtering. This way it
won't use filtered out decls to hide potential candidates.

rdar://148286345
---
 lib/Serialization/Deserialization.cpp         |  6 +++
 .../shadowed-class-vs-protocol.swift          | 50 +++++++++++++++++++
 2 files changed, 56 insertions(+)
 create mode 100644 test/Serialization/shadowed-class-vs-protocol.swift

diff --git a/lib/Serialization/Deserialization.cpp b/lib/Serialization/Deserialization.cpp
index d1f0d96b1c9..52589b526e0 100644
--- a/lib/Serialization/Deserialization.cpp
+++ b/lib/Serialization/Deserialization.cpp
@@ -2766,6 +2766,12 @@ giveUpFastPath:
   if (M)
     return diagnoseFatal();
 
+  if (values.size() > 1) {
+    // Apply shadowing filtering after other local filters so we don't rule out
+    // valid candidates shadowed by invalid ones.
+    removeShadowedDecls(values, baseModule);
+  }
+
   // When all is said and done, we should have a single value here to return.
   if (values.size() != 1) {
     return llvm::make_error<XRefError>("result is ambiguous", pathTrace,
diff --git a/test/Serialization/shadowed-class-vs-protocol.swift b/test/Serialization/shadowed-class-vs-protocol.swift
new file mode 100644
index 00000000000..04d3004233a
--- /dev/null
+++ b/test/Serialization/shadowed-class-vs-protocol.swift
@@ -0,0 +1,50 @@
+/// Ensure cross references to shadowed decls take into account the shadowing
+/// after the custom deserialization filtering.
+
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// REQUIRES: objc_interop
+
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) \
+// RUN:   %t/SwiftLib.swift -I %t -emit-module-path %t/SwiftLib.swiftmodule
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) \
+// RUN:   -typecheck %t/Client.swift -I %t
+
+//--- module.modulemap
+module RootLib {
+  header "RootLib.h"
+}
+
+module MiddleLib {
+  header "MiddleLib.h"
+  export *
+}
+
+//--- RootLib.h
+__attribute__((swift_name("ShadowedProtocol")))
+@protocol Shadowed
+@end
+
+//--- MiddleLib.h
+@import Foundation;
+#include "RootLib.h"
+
+@interface Shadowed: NSObject <Shadowed>
+@end
+
+//--- SwiftLib.swift
+import MiddleLib
+
+public func funcRef() -> Shadowed { fatalError() }
+
+extension Shadowed {
+    public func method() -> Shadowed { fatalError() }
+}
+
+//--- Client.swift
+import SwiftLib
+
+@inlinable
+public func bar() {
+    let _ = funcRef().method()
+}