diff --git a/flang/lib/Optimizer/CodeGen/CodeGen.cpp b/flang/lib/Optimizer/CodeGen/CodeGen.cpp
--- a/flang/lib/Optimizer/CodeGen/CodeGen.cpp
+++ b/flang/lib/Optimizer/CodeGen/CodeGen.cpp
@@ -13,8 +13,10 @@
 #include "flang/Optimizer/CodeGen/CodeGen.h"
 #include "PassDetail.h"
 #include "flang/ISO_Fortran_binding.h"
+#include "flang/Optimizer/Builder/FIRBuilder.h"
 #include "flang/Optimizer/Dialect/FIRAttr.h"
 #include "flang/Optimizer/Dialect/FIROps.h"
+#include "flang/Optimizer/Support/InternalNames.h"
 #include "mlir/Conversion/ArithmeticToLLVM/ArithmeticToLLVM.h"
 #include "mlir/Conversion/LLVMCommon/Pattern.h"
 #include "mlir/Conversion/StandardToLLVM/ConvertStandardToLLVM.h"
@@ -636,6 +638,18 @@
   }
 };
 
+/// Lower `fir.gentypedesc` to a global constant.
+struct GenTypeDescOpConversion : public FIROpConversion<fir::GenTypeDescOp> {
+  using FIROpConversion::FIROpConversion;
+
+  mlir::LogicalResult
+  matchAndRewrite(fir::GenTypeDescOp gentypedesc, OpAdaptor adaptor,
+                  mlir::ConversionPatternRewriter &rewriter) const override {
+    return rewriter.notifyMatchFailure(
+        gentypedesc, "fir.fir.gentypedesc codegen is not implemented yet");
+  }
+};
+
 /// Lower `fir.has_value` operation to `llvm.return` operation.
 struct HasValueOpConversion : public FIROpConversion<fir::HasValueOp> {
   using FIROpConversion::FIROpConversion;
@@ -1417,12 +1431,13 @@
         BoxIsPtrOpConversion, BoxRankOpConversion, CallOpConversion,
         ConvertOpConversion, DispatchOpConversion, DispatchTableOpConversion,
         DTEntryOpConversion, DivcOpConversion, ExtractValueOpConversion,
-        HasValueOpConversion, GlobalOpConversion, InsertOnRangeOpConversion,
-        InsertValueOpConversion, IsPresentOpConversion, LoadOpConversion,
-        NegcOpConversion, MulcOpConversion, SelectCaseOpConversion,
-        SelectOpConversion, SelectRankOpConversion, StoreOpConversion,
-        SubcOpConversion, UndefOpConversion, UnreachableOpConversion,
-        ZeroOpConversion>(typeConverter);
+        HasValueOpConversion, GenTypeDescOpConversion, GlobalOpConversion,
+        InsertOnRangeOpConversion, InsertValueOpConversion,
+        IsPresentOpConversion, LoadOpConversion, NegcOpConversion,
+        MulcOpConversion, SelectCaseOpConversion, SelectOpConversion,
+        SelectRankOpConversion, StoreOpConversion, SubcOpConversion,
+        UndefOpConversion, UnreachableOpConversion, ZeroOpConversion>(
+        typeConverter);
     mlir::populateStdToLLVMConversionPatterns(typeConverter, pattern);
     mlir::arith::populateArithmeticToLLVMConversionPatterns(typeConverter,
                                                             pattern);
diff --git a/flang/test/Fir/convert-to-llvm-invalid.fir b/flang/test/Fir/convert-to-llvm-invalid.fir
--- a/flang/test/Fir/convert-to-llvm-invalid.fir
+++ b/flang/test/Fir/convert-to-llvm-invalid.fir
@@ -50,3 +50,13 @@
 ^bb3:
   return
 }
+
+// -----
+
+// Test `fir.gentypedesc` conversion failure. Not implemented yet.
+
+func @gentypedesc() {
+  // expected-error@+1{{failed to legalize operation 'fir.gentypedesc'}}
+  %0 = fir.gentypedesc !fir.type<derived3>
+  return
+}