diff --git a/clang/docs/AutomaticReferenceCounting.rst b/clang/docs/AutomaticReferenceCounting.rst
--- a/clang/docs/AutomaticReferenceCounting.rst
+++ b/clang/docs/AutomaticReferenceCounting.rst
@@ -1863,6 +1863,12 @@
 ``Block_copy``.  The optimizer may remove such copies when it sees that the
 result is used only as an argument to a call.
 
+When a block pointer type is converted to type ``id``, ``Block_copy`` is called.
+This is necessary because a block allocated on the stack won't get copied to the
+heap when the pointer converted to ``id`` escapes. The conversion to ``id``
+implicitly takes place when a block is passed to a function as a variadic
+argument.
+
 .. _arc.misc.exceptions:
 
 Exceptions
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -5247,6 +5247,9 @@
       for (Expr *A : Args.slice(ArgIx)) {
         ExprResult Arg = DefaultVariadicArgumentPromotion(A, CallType, FDecl);
         Invalid |= Arg.isInvalid();
+        // Copy blocks to the heap.
+        if (A->getType()->isBlockPointerType())
+          maybeExtendBlockObject(Arg);
         AllArgs.push_back(Arg.get());
       }
     }
diff --git a/clang/test/CodeGenObjC/arc-blocks.m b/clang/test/CodeGenObjC/arc-blocks.m
--- a/clang/test/CodeGenObjC/arc-blocks.m
+++ b/clang/test/CodeGenObjC/arc-blocks.m
@@ -730,5 +730,15 @@
   test20_callee(^{ (void)x; });
 }
 
+// CHECK-LABEL: define void @test21(
+// CHECK: %[[V6:.*]] = call i8* @llvm.objc.retainBlock(
+// CHECK: %[[V7:.*]] = bitcast i8* %[[V6]] to void ()*
+// CHECK: call void (i32, ...) @test21_callee(i32 1, void ()* %[[V7]]),
+
+void test21_callee(int n, ...);
+void test21(id x) {
+  test21_callee(1, ^{ (void)x; });
+}
+
 // CHECK: attributes [[NUW]] = { nounwind }
 // CHECK-UNOPT: attributes [[NUW]] = { nounwind }