Sema: Effectively revert #70569.

The fix caused a source break that is now captured as an additional test case
in `objc_async_conformance.swift`.

Resolves rdar://121527977

Author: tshortli

lib/Sema/TypeCheckProtocol.cpp

@@ -4943,6 +4943,8 @@ hasInvalidTypeInConformanceContext(const ValueDecl *requirement,
 
 void ConformanceChecker::resolveValueWitnesses() {
   bool usesPreconcurrencyConformance = false;
+  auto objcRequirementMap = getObjCRequirementMap(Proto);
+
   for (auto *requirement : Proto->getProtocolRequirements()) {
     // Associated type requirements handled elsewhere.
     if (isa<TypeDecl>(requirement))
@@ -5080,6 +5082,23 @@ void ConformanceChecker::resolveValueWitnesses() {
       continue;
     }
 
+    // If this requirement is part of a pair of imported async requirements,
+    // where one has already been witnessed, we can skip it.
+    //
+    // This situation primarily arises when the ClangImporter translates an
+    // async-looking ObjC protocol method requirement into two Swift protocol
+    // requirements: an async version and a sync version. Exactly one of the two
+    // must be witnessed by the conformer.
+    if (!requirement->isImplicit() &&
+        getObjCRequirementSibling(
+            Proto, requirement, objcRequirementMap,
+            [this](AbstractFunctionDecl *cand) {
+              return !cand->getAttrs().hasAttribute<OptionalAttr>() &&
+                     !cand->isImplicit() && this->Conformance->hasWitness(cand);
+            })) {
+      continue;
+    }
+
     // Try substituting into the requirement's interface type. If we fail,
     // either a generic requirement was not satisfied or we tripped on an
     // invalid type witness, and there's no point in resolving a witness.

test/ClangImporter/objc_async_conformance.swift

@@ -112,3 +112,7 @@ class C5 {
 class C6: C5, ServiceProvider {
   @MainActor func allOperations() async -> [String] { [] }
 }
+
+extension ImplementsLoadable: @retroactive Loadable {
+  public func loadStuff(withOtherIdentifier otherIdentifier: Int, reply: @escaping () -> Void) {}
+}

test/Inputs/clang-importer-sdk/usr/include/ObjCConcurrency.h

@@ -344,4 +344,15 @@ MAIN_ACTOR
                                                    error:(NSError * __autoreleasing *)error;
 @end
 
+@protocol Loadable
+- (void)loadStuffWithIdentifier:(NSInteger)identifier reply:(void (^)())reply;
+- (void)loadStuffWithOtherIdentifier:(NSInteger)otherIdentifier reply:(void (^)())reply;
+- (void)loadStuffWithGroupID:(NSInteger)groupID reply:(void (^)())reply;
+@end
+
+@interface ImplementsLoadable : NSObject
+- (void)loadStuffWithIdentifier:(NSInteger)identifier reply:(void (^)())reply;
+- (void)loadStuffWithGroupID:(NSInteger)groupID reply:(void (^)())reply;
+@end
+
 #pragma clang assume_nonnull end

test/Serialization/conformance-objc-async.swift

@@ -4,6 +4,8 @@
 // RUN: %target-swift-frontend -compile-module-from-interface %t/Conformance.swiftinterface -module-name Conformance -o /dev/null -I %t
 // REQUIRES: objc_interop
 
+// REQUIRES: rdar119435253
+
 //--- module.modulemap
 module ObjCProto {
   header "objc_proto.h"