diff --git a/mlir/include/mlir/IR/OpDefinition.h b/mlir/include/mlir/IR/OpDefinition.h
--- a/mlir/include/mlir/IR/OpDefinition.h
+++ b/mlir/include/mlir/IR/OpDefinition.h
@@ -93,8 +93,8 @@
 /// they aren't customized.
 class OpState {
 public:
-  /// Ops are pointer-like, so we allow implicit conversion to bool.
-  operator bool() { return getOperation() != nullptr; }
+  /// Ops are pointer-like, so we allow conversion to bool.
+  explicit operator bool() { return getOperation() != nullptr; }
 
   /// This implicitly converts to Operation*.
   operator Operation *() const { return state; }