diff --git a/flang/lib/Lower/ConvertExpr.cpp b/flang/lib/Lower/ConvertExpr.cpp
index e4ccddc84f1cb..90de4b83216dd 100644
--- a/flang/lib/Lower/ConvertExpr.cpp
+++ b/flang/lib/Lower/ConvertExpr.cpp
@@ -3785,6 +3785,12 @@ class ArrayExprLowering {
   /// Lower rhs of an array expression.
   ExtValue lowerArrayExpression(const Fortran::lower::SomeExpr &exp) {
     mlir::Type resTy = converter.genType(exp);
+
+    if (fir::isPolymorphicType(resTy) &&
+        Fortran::evaluate::HasVectorSubscript(exp))
+      TODO(getLoc(),
+           "polymorphic array expression lowering with vector subscript");
+
     return std::visit(
         [&](const auto &e) { return lowerArrayExpression(genarr(e), resTy); },
         exp.u);