Partially revert “Don’t declare async methods with alternatives in headers”

This reverts parts of commit d4ee34c. Specifically, it reverts the implementation, implements a different workaround for the bug, and modifies the test to pass if the workaround is effective.

Fixes rdar://97810819.

Author: beccadax

lib/PrintAsClang/DeclAndTypePrinter.cpp

@@ -2019,31 +2019,12 @@ auto DeclAndTypePrinter::getImpl() -> Implementation {
   return Implementation(os, *this, outputLang);
 }
 
-static bool isAsyncAlternativeOfOtherDecl(const ValueDecl *VD) {
-  auto AFD = dyn_cast<AbstractFunctionDecl>(VD);
-  if (!AFD || !AFD->isAsyncContext() || !AFD->getObjCSelector())
-    return false;
-
-  auto type = AFD->getDeclContext()->getSelfNominalTypeDecl();
-  if (!type)
-    return false;
-  auto others = type->lookupDirect(AFD->getObjCSelector(),
-                                   AFD->isInstanceMember());
-
-  for (auto other : others)
-    if (other->getAsyncAlternative() == AFD)
-      return true;
-
-  return false;
-}
-
 bool DeclAndTypePrinter::shouldInclude(const ValueDecl *VD) {
   return !VD->isInvalid() &&
          (outputLang == OutputLanguageMode::Cxx
               ? cxx_translation::isVisibleToCxx(VD, minRequiredAccess)
               : isVisibleToObjC(VD, minRequiredAccess)) &&
-         !VD->getAttrs().hasAttribute<ImplementationOnlyAttr>() &&
-         !isAsyncAlternativeOfOtherDecl(VD);
+         !VD->getAttrs().hasAttribute<ImplementationOnlyAttr>();
 }
 
 void DeclAndTypePrinter::print(const Decl *D) {

lib/PrintAsClang/PrintAsClang.cpp

@@ -80,6 +80,7 @@ static void writePrologue(raw_ostream &out, ASTContext &ctx,
          "# include <swift/objc-prologue.h>\n"
          "#endif\n"
          "\n"
+         "#pragma clang diagnostic ignored \"-Wduplicate-method-match\"\n"
          "#pragma clang diagnostic ignored \"-Wauto-import\"\n";
   emitObjCConditional(out,
                       [&] { out << "#include <Foundation/Foundation.h>\n"; });

test/PrintAsObjC/protocols.swift

@@ -28,6 +28,8 @@ import objc_generics
 @objc protocol B : A, Sendable {}
 
 // CHECK-LABEL: @protocol CompletionAndAsync
+// FIXME: We should detect this duplication.
+// CHECK-NEXT: - (void)helloWithCompletion:(void (^ _Nonnull)(BOOL))completionHandler SWIFT_AVAILABILITY
 // CHECK-NEXT: - (void)helloWithCompletion:(void (^ _Nonnull)(BOOL))completion;
 // CHECK-NEXT: @end
 @objc protocol CompletionAndAsync {