[NameLookup] Shadowing: strip @Sendable annotations for declarations ObjC/@objc declarations

When `SendableCompletionHandlers` feature is enabled, ClangImporter
is going to inject `@Sendable` attribute into the completion handler
parameter's function type. We need to make sure that his new behavior
doesn't break shadowing rules and allow APIs to annotate their ObjC
declarations with `@Sendable` without breaking clients.

Author: xedin

lib/AST/NameLookup.cpp

@@ -826,6 +826,20 @@ static void recordShadowedDeclsAfterSignatureMatch(
     else
       type = removeThrownError(decl->getInterfaceType()->getCanonicalType());
 
+    // Strip `@Sendable` annotations for declarations that come from
+    // or are exposed to Objective-C to make it possible to introduce
+    // new annotations without breaking shadowing rules.
+    //
+    // This is a narrow fix for specific backwards-incompatible cases
+    // we know about, it could be extended to use `isObjC()` in the
+    // future if we find problematic cases were a more general change
+    // is required.
+    if (decl->hasClangNode() || decl->getAttrs().hasAttribute<ObjCAttr>()) {
+      type =
+          type->stripConcurrency(/*recursive=*/true, /*dropGlobalActor=*/false)
+              ->getCanonicalType();
+    }
+
     // Record this declaration based on its signature.
     auto &known = collisions[type];
     if (known.size() == 1) {

test/Concurrency/sendable_objc_attr_in_type_context_swift5.swift

@@ -69,6 +69,11 @@ void doSomethingConcurrently(__attribute__((noescape)) void SWIFT_SENDABLE (^blo
 -(void)computeWithCompletionHandler: (void (^)(void)) handler;
 @end
 
+@interface Shadowing : NSObject
+-(void)computeWithCompletion: (void (^)(void)) completion;
+-(void)updateWithCompletionHandler: (void (^_Nullable)(void)) handler;
+@end
+
 #pragma clang assume_nonnull end
 
 //--- main.swift
@@ -158,3 +163,16 @@ class TestConformanceWithoutStripping : InnerSendableTypes {
   @objc func compute(completionHandler: @escaping () -> Void) {}
   // expected-warning@-1 {{sendability of function types in instance method 'compute(completionHandler:)' of type '(@escaping () -> Void) -> ()' does not match type '(@escaping @Sendable () -> Void) -> Void' declared by the header}}
 }
+
+// Methods deliberately has no `@Sendable` to make sure that
+// shadowing rules are preserved when SendableCompletionHandlers feature is enabled.
+@objc extension Shadowing {
+  @objc func compute(completion: @escaping () -> Void) {}
+
+  @objc func update(completionHandler: (() -> Void)? = nil) {}
+
+  func testCompute() {
+    self.compute { } // Ok - no ambiguity
+    self.update { }  // Ok - no ambiguity
+  }
+}

test/Concurrency/sendable_objc_attr_in_type_context_swift6.swift

@@ -69,6 +69,11 @@ void doSomethingConcurrently(__attribute__((noescape)) void SWIFT_SENDABLE (^blo
 -(void)computeWithCompletionHandler: (void (^)(void)) handler;
 @end
 
+@interface Shadowing : NSObject
+-(void)computeWithCompletion: (void (^)(void)) completion;
+-(void)updateWithCompletionHandler: (void (^_Nullable)(void)) handler;
+@end
+
 #pragma clang assume_nonnull end
 
 //--- main.swift
@@ -165,3 +170,16 @@ class TestConformanceWithoutStripping : InnerSendableTypes {
   @objc func compute(completionHandler: @escaping () -> Void) {}
   // expected-warning@-1 {{sendability of function types in instance method 'compute(completionHandler:)' of type '(@escaping () -> Void) -> ()' does not match type '(@escaping @Sendable () -> Void) -> Void' declared by the header}}
 }
+
+// Methods deliberately has no `@Sendable` to make sure that
+// shadowing rules are preserved when SendableCompletionHandlers feature is enabled.
+@objc extension Shadowing {
+  @objc func compute(completion: @escaping () -> Void) {}
+
+  @objc func update(completionHandler: (() -> Void)? = nil) {}
+
+  func testCompute() {
+    self.compute { } // Ok - no ambiguity
+    self.update { }  // Ok - no ambiguity
+  }
+}