diff --git a/bolt/lib/Passes/BinaryPasses.cpp b/bolt/lib/Passes/BinaryPasses.cpp
--- a/bolt/lib/Passes/BinaryPasses.cpp
+++ b/bolt/lib/Passes/BinaryPasses.cpp
@@ -147,6 +147,9 @@
                     cl::desc("dump CFG of functions with unknown control flow"),
                     cl::cat(BoltCategory), cl::ReallyHidden);
 
+// Please MSVC19 with a forward declaration: otherwise it reports an error about
+// an undeclared variable inside a callback.
+extern cl::opt<bolt::ReorderBasicBlocks::LayoutType> ReorderBlocks;
 cl::opt<bolt::ReorderBasicBlocks::LayoutType> ReorderBlocks(
     "reorder-blocks", cl::desc("change layout of basic blocks in a function"),
     cl::init(bolt::ReorderBasicBlocks::LT_NONE),