Index: clang/include/clang/Basic/Attr.td
===================================================================
--- clang/include/clang/Basic/Attr.td
+++ clang/include/clang/Basic/Attr.td
@@ -2331,6 +2331,16 @@
   let Documentation = [SwiftIndirectResultDocs];
 }
 
+def SwiftAsync : InheritableAttr {
+  let Spellings = [Clang<"swift_async">];
+  let Subjects = SubjectList<[Function, ObjCMethod]>;
+  let Args = [EnumArgument<"Kind", "Kind",
+                ["none", "swift_private", "not_swift_private"],
+                ["None", "SwiftPrivate", "NotSwiftPrivate"]>,
+              ParamIdxArgument<"CompletionHandlerIndex", /*opt=*/1>];
+  let Documentation = [SwiftAsyncDocs];
+}
+
 def Suppress : StmtAttr {
   let Spellings = [CXX11<"gsl", "suppress">];
   let Args = [VariadicStringArgument<"DiagnosticIdentifiers">];
Index: clang/include/clang/Basic/AttrDocs.td
===================================================================
--- clang/include/clang/Basic/AttrDocs.td
+++ clang/include/clang/Basic/AttrDocs.td
@@ -4343,6 +4343,36 @@
   }];
 }
 
+def SwiftAsyncDocs : Documentation {
+  let Category = SwiftDocs;
+  let Heading = "swift_async";
+  let Content = [{
+The ``swift_async`` attribute specifies if and how a particular function or
+Objective-C method is imported into a swift async method. For instance:
+
+.. code-block:: objc
+
+  @interface MyClass : NSObject
+  -(void)notActuallyAsync:(int)p1 withCompletionHandler:(void (^)())handler
+      __attribute__((swift_async(none)));
+
+  -(void)actuallyAsync:(int)p1 callThisAsync:(void (^)())fun
+      __attribute__((swift_async(swift_private, 1)));
+  @end
+
+Here, ``notActuallyAsync:withCompletionHandler`` would have been imported as
+``async`` (because it's last parameter's selector piece is
+``withCompletionHandler``) if not for ``swift_async(none)``. Conversely,
+``actuallyAsync:callThisAsync`` wouldn't have been imported as ``async`` if not
+for ``swift_async`` because it doesn't match the naming convention.
+
+When using ``swift_async`` to enable importing, the first argument to the
+attribute is either ``swift_private`` or ``not_swift_private`` to indicate
+whether the function/method is private to the current framework, and the second
+argument is the index of the completion handler parameter.
+  }];
+}
+
 def SuppressDocs : Documentation {
   let Category = DocCatStmt;
   let Content = [{
Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -4038,6 +4038,13 @@
   "%0 attribute with '%1' convention can only be applied to a "
   "%select{function|method}2 returning %select{an integral type|a pointer}3">;
 
+def err_swift_async_no_access : Error<
+  "first argument to 'swift_async' must be either 'none', 'swift_private', or "
+  "'not_swift_private'">;
+def err_swift_async_bad_block_type : Error<
+  "'swift_async' completion handler parameter must have block type returning"
+  " 'void', type here is %0">;
+
 def warn_ignored_objc_externally_retained : Warning<
   "'objc_externally_retained' can only be applied to local variables "
   "%select{of retainable type|with strong ownership}0">,
Index: clang/lib/Sema/SemaDeclAttr.cpp
===================================================================
--- clang/lib/Sema/SemaDeclAttr.cpp
+++ clang/lib/Sema/SemaDeclAttr.cpp
@@ -6033,6 +6033,56 @@
   D->addAttr(::new (S.Context) SwiftNewTypeAttr(S.Context, AL, Kind));
 }
 
+static void handleSwiftAsyncAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+  if (!AL.isArgIdent(0)) {
+    S.Diag(AL.getLoc(), diag::err_attribute_argument_n_type)
+        << AL << 1 << AANT_ArgumentIdentifier;
+    return;
+  }
+
+  SwiftAsyncAttr::Kind Kind;
+  IdentifierInfo *II = AL.getArgAsIdent(0)->Ident;
+  if (!SwiftAsyncAttr::ConvertStrToKind(II->getName(), Kind)) {
+    S.Diag(AL.getLoc(), diag::err_swift_async_no_access) << AL << II;
+    return;
+  }
+
+  ParamIdx Idx;
+  if (Kind == SwiftAsyncAttr::None) {
+    // If this is 'none', then there shouldn't be any additional arguments.
+    if (!checkAttributeNumArgs(S, AL, 1))
+      return;
+  } else {
+    // Non-none swift_async requires a completion handler index argument.
+    if (!checkAttributeNumArgs(S, AL, 2))
+      return;
+
+    Expr *HandlerIdx = AL.getArgAsExpr(1);
+    if (!checkFunctionOrMethodParameterIndex(S, D, AL, 2, HandlerIdx, Idx))
+      return;
+
+    const ParmVarDecl *CompletionBlock =
+        getFunctionOrMethodParam(D, Idx.getASTIndex());
+    QualType CompletionBlockType = CompletionBlock->getType();
+    if (!CompletionBlockType->isBlockPointerType()) {
+      S.Diag(CompletionBlock->getLocation(),
+             diag::err_swift_async_bad_block_type)
+          << CompletionBlock->getType();
+      return;
+    }
+    QualType BlockTy =
+        CompletionBlockType->getAs<BlockPointerType>()->getPointeeType();
+    if (!BlockTy->getAs<FunctionType>()->getReturnType()->isVoidType()) {
+      S.Diag(CompletionBlock->getLocation(),
+             diag::err_swift_async_bad_block_type)
+          << CompletionBlock->getType();
+      return;
+    }
+  }
+
+  D->addAttr(::new (S.Context) SwiftAsyncAttr(S.Context, AL, Kind, Idx));
+}
+
 //===----------------------------------------------------------------------===//
 // Microsoft specific attribute handlers.
 //===----------------------------------------------------------------------===//
@@ -7962,6 +8012,9 @@
   case ParsedAttr::AT_SwiftPrivate:
     handleSimpleAttribute<SwiftPrivateAttr>(S, D, AL);
     break;
+  case ParsedAttr::AT_SwiftAsync:
+    handleSwiftAsyncAttr(S, D, AL);
+    break;
 
   // XRay attributes.
   case ParsedAttr::AT_XRayLogArgs:
Index: clang/test/Misc/pragma-attribute-supported-attributes-list.test
===================================================================
--- clang/test/Misc/pragma-attribute-supported-attributes-list.test
+++ clang/test/Misc/pragma-attribute-supported-attributes-list.test
@@ -147,6 +147,7 @@
 // CHECK-NEXT: Section (SubjectMatchRule_function, SubjectMatchRule_variable_is_global, SubjectMatchRule_objc_method, SubjectMatchRule_objc_property)
 // CHECK-NEXT: SetTypestate (SubjectMatchRule_function_is_member)
 // CHECK-NEXT: SpeculativeLoadHardening (SubjectMatchRule_function, SubjectMatchRule_objc_method)
+// CHECK-NEXT: SwiftAsync (SubjectMatchRule_function, SubjectMatchRule_objc_method)
 // CHECK-NEXT: SwiftBridgedTypedef (SubjectMatchRule_type_alias)
 // CHECK-NEXT: SwiftContext (SubjectMatchRule_variable_is_parameter)
 // CHECK-NEXT: SwiftError (SubjectMatchRule_function, SubjectMatchRule_objc_method)
Index: clang/test/SemaObjC/attr-swift-async.m
===================================================================
--- /dev/null
+++ clang/test/SemaObjC/attr-swift-async.m
@@ -0,0 +1,38 @@
+// RUN: %clang_cc1                 -verify -fsyntax-only -fblocks %s
+// RUN: %clang_cc1 -xobjective-c++ -verify -fsyntax-only -fblocks %s
+
+#define SA(...) __attribute__((swift_async(__VA_ARGS__)))
+
+SA(none) int a; // expected-warning{{'swift_async' attribute only applies to functions and Objective-C methods}}
+
+SA(none) void b();
+
+SA(not_swift_private, 0) void c(); // expected-error{{'swift_async' attribute parameter 2 is out of bounds}}
+SA(swift_private, 1) void d(); // expected-error{{'swift_async' attribute parameter 2 is out of bounds}}
+SA(swift_private, 1) void e(int); // expected-error{{'swift_async' completion handler parameter must have block type returning 'void', type here is 'int'}}
+SA(not_swift_private, 1) void f(int (^)()); // expected-error{{'swift_async' completion handler parameter must have block type returning 'void', type here is 'int (^)()'}}
+SA(swift_private, 1) void g(void (^)());
+
+SA(none, 1) void h(); // expected-error{{'swift_async' attribute takes one argument}}
+SA() void i(); // expected-error{{'swift_async' attribute takes at least 1 argument}}
+SA(not_swift_private) void j(); // expected-error{{'swift_async' attribute requires exactly 2 arguments}}
+SA(43) void k(); // expected-error{{'swift_async' attribute requires parameter 1 to be an identifier}}
+SA(not_a_thing, 0) void l(); // expected-error{{first argument to 'swift_async' must be either 'none', 'swift_private', or 'not_swift_private'}}
+
+@interface TestOnMethods
+-(void)m1:(int (^)())callback SA(swift_private, 1); // expected-error{{'swift_async' completion handler parameter must have block type returning 'void', type here is 'int (^)()'}}
+-(void)m2:(void (^)())callback SA(swift_private, 0); // expected-error{{'swift_async' attribute parameter 2 is out of bounds}}
+-(void)m3:(void (^)())callback SA(swift_private, 2); // expected-error{{'swift_async' attribute parameter 2 is out of bounds}}
+-(void)m4 SA(none);
+-(void)m5:(int)p handler:(void (^)(int))callback SA(not_swift_private, 2);
+@end
+
+#ifdef __cplusplus
+struct S {
+  SA(none) void mf1();
+  SA(swift_private, 2) void mf2(void (^)());
+  SA(swift_private, 1) void mf3(void (^)()); // expected-error{{'swift_async' attribute is invalid for the implicit this argument}}
+  SA(swift_private, 0) void mf4(void (^)()); // expected-error{{'swift_async' attribute parameter 2 is out of bounds}}
+  SA(not_swift_private, 2) void mf5(int (^)()); // expected-error{{'swift_async' completion handler parameter must have block type returning 'void', type here is 'int (^)()'}}
+};
+#endif