Serialization: Ignore lookup shadowing when resolving cross-references

Inlined code may refer to the shadowed or the shadower. Typical
shadowing is already handled by the cross-reference lookup logic
which looks for the defining module, scope and wether it's a Swift or
clang decl. We can disable the lookup shadowing logic and rely only on
the custom filtering.

rdar://146320871
#79801

Author: xymus

lib/Serialization/Deserialization.cpp

@@ -2075,7 +2075,7 @@ ModuleFile::resolveCrossReference(ModuleID MID, uint32_t pathLen) {
                                getIdentifier(privateDiscriminator));
     } else {
       baseModule->lookupQualified(baseModule, DeclNameRef(name),
-                                  SourceLoc(), NL_QualifiedDefault,
+                                  SourceLoc(), NL_RemoveOverridden,
                                   values);
     }
     filterValues(filterTy, nullptr, nullptr, isType, inProtocolExt,
@@ -2285,8 +2285,8 @@ ModuleFile::resolveCrossReference(ModuleID MID, uint32_t pathLen) {
                                    getIdentifier(privateDiscriminator));
         } else {
           otherModule->lookupQualified(otherModule, DeclNameRef(name),
-                                      SourceLoc(), NL_QualifiedDefault,
-                                      values);
+                                       SourceLoc(), NL_RemoveOverridden,
+                                       values);
         }
 
         std::optional<ValueDecl*> matchBeforeFiltering = std::nullopt;

test/Serialization/inlined-shadowed.swift

@@ -0,0 +1,60 @@
+/// Ensure inlined code in swiftmodules differentiates between shadowed Swift
+/// and clang decls.
+
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// REQUIRES: objc_interop
+
+/// Reference build: Build libs and client for a normal build.
+// RUN: %target-swift-frontend -emit-module -O %t/RootLib.swift -I %t \
+// RUN:   -emit-module-path %t/RootLib.swiftmodule
+// RUN: %target-swift-frontend -emit-module -O %t/MiddleLib.swift -I %t \
+// RUN:   -emit-module-path %t/MiddleLib.swiftmodule
+// RUN: %target-swift-frontend -emit-sil -O %t/Client.swift -I %t > %t/Client.sil
+// RUN: %FileCheck %s --input-file=%t/Client.sil
+
+//--- module.modulemap
+module RootLib {
+  header "RootLib.h"
+}
+
+//--- RootLib.h
+#include <stdio.h>
+void ShadowedFunc() {
+    printf("Clang\n");
+}
+
+//--- RootLib.swift
+@_exported import RootLib
+
+@usableFromInline
+internal func ShadowedFunc() {
+    print("Swift")
+}
+
+@inlinable
+@inline(__always)
+public func swiftUser() {
+    ShadowedFunc() // Swift one
+}
+
+//--- MiddleLib.swift
+
+import RootLib
+
+@inlinable
+@inline(__always)
+public func clangUser() {
+    ShadowedFunc() // Clang one
+}
+
+//--- Client.swift
+
+import RootLib
+import MiddleLib
+
+swiftUser()
+// CHECK: function_ref @$s7RootLib12ShadowedFuncyyF : $@convention(thin) () -> ()
+
+clangUser()
+// CHECK: function_ref @ShadowedFunc : $@convention(c) () -> ()