[TypeChecker] Ignore mismatches on sendability in @objc @implementation witnesses unless in strict concurrency mode

The requirement is `@preconcurrency` in this case and the checking
should abide by the rules.

Author: xedin

lib/Sema/TypeCheckDeclObjC.cpp

@@ -3889,12 +3889,27 @@ class ObjCImplementationChecker {
       diagnoseVTableUse(cand);
       return;
 
-    case MatchOutcome::WrongSendability:
+    case MatchOutcome::WrongSendability: {
+      auto concurrencyLevel = req->getASTContext().LangOpts.StrictConcurrencyLevel;
+
+      DiagnosticBehavior behavior;
+      switch (concurrencyLevel) {
+      case StrictConcurrency::Complete:
+        behavior = DiagnosticBehavior::Warning;
+        break;
+
+      case StrictConcurrency::Targeted:
+      case StrictConcurrency::Minimal:
+        behavior = DiagnosticBehavior::Ignore;
+        break;
+      }
+
       diagnose(cand, diag::objc_implementation_sendability_mismatch, cand,
                getMemberType(cand), getMemberType(req))
-          .limitBehaviorWithPreconcurrency(DiagnosticBehavior::Warning,
+          .limitBehaviorWithPreconcurrency(behavior,
                                            /*preconcurrency*/ true);
       return;
+    }
 
     case MatchOutcome::WrongImplicitObjCName:
     case MatchOutcome::WrongExplicitObjCName: {

test/Concurrency/sendable_objc_attr_in_type_context_swift5.swift

@@ -172,8 +172,7 @@ class TestConformanceWithoutStripping : InnerSendableTypes {
   @objc public required init(callback: @escaping () -> Void) {}
   // expected-error@-1 {{initializer 'init(callback:)' should not be 'required' to match initializer declared by the header}}
 
-  @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}}
+  @objc func compute(completionHandler: @escaping () -> Void) {} // Ok (no warnings with minimal checking)
 }
 
 // Methods deliberately has no `@Sendable` to make sure that

test/Concurrency/sendable_objc_attr_in_type_context_swift5_strict.swift

@@ -65,6 +65,11 @@ void doSomethingConcurrently(__attribute__((noescape)) void SWIFT_SENDABLE (^blo
 -(void) testWithCallback:(NSString *)name handler:(MAIN_ACTOR void (^)(NSDictionary<NSString *, SWIFT_SENDABLE id> *, NSError * _Nullable))handler;
 @end
 
+@interface SwiftImpl : NSObject
+-(id)initWithCallback:  (void (^ SWIFT_SENDABLE)(void)) callback;
+-(void)computeWithCompletionHandler: (void (^)(void)) handler;
+@end
+
 #pragma clang assume_nonnull end
 
 //--- main.swift
@@ -153,3 +158,11 @@ class TestConformanceWithoutStripping : InnerSendableTypes {
   func test(withCallback name: String, handler: @escaping @MainActor ([String : any Sendable], (any Error)?) -> Void) { // Ok
   }
 }
+
+@objc @implementation extension SwiftImpl {
+  @objc public required init(callback: @escaping () -> Void) {}
+  // expected-error@-1 {{initializer 'init(callback:)' should not be 'required' to match initializer declared by the header}}
+  
+  @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}}
+}