Check correct dump format for gblinear. (#10831)

src/gbm/gblinear_model.h

@@ -125,7 +125,7 @@ class GBLinearModel : public Model {
         }
       }
       fo << std::endl << "    ]" << std::endl << "  }";
-    } else {
+    } else if (format == "text") {
       fo << "bias:\n";
       for (int gid = 0; gid < ngroup; ++gid) {
         fo << this->Bias()[gid] << std::endl;
@@ -136,6 +136,8 @@ class GBLinearModel : public Model {
           fo << (*this)[i][gid] << std::endl;
         }
       }
+    } else {
+      LOG(FATAL) << "Dump format `" << format << "` is not supported by the gblinear model.";
     }
     std::vector<std::string> v;
     v.push_back(fo.str());

tests/cpp/gbm/test_gblinear.cc

@@ -1,21 +1,18 @@
-/*!
- * Copyright 2019 by Contributors
+/**
+ * Copyright 2019-2024, XGBoost Contributors
  */
 #include <gtest/gtest.h>
+#include <xgboost/feature_map.h>  // for FeatureMap
 
 #include <memory>
-#include <sstream>
 
 #include "../helpers.h"
 #include "xgboost/context.h"
 #include "xgboost/gbm.h"
 #include "xgboost/json.h"
 #include "xgboost/learner.h"
-#include "xgboost/logging.h"
-
-namespace xgboost {
-namespace gbm {
 
+namespace xgboost::gbm {
 TEST(GBLinear, JsonIO) {
   size_t constexpr kRows = 16, kCols = 16;
 
@@ -40,5 +37,16 @@ TEST(GBLinear, JsonIO) {
     ASSERT_EQ(weights.size(), 17);
   }
 }
-}  // namespace gbm
-}  // namespace xgboost
+
+TEST(GBLinear, Dump) {
+  Context ctx;
+  size_t constexpr kRows = 16, kCols = 16;
+  LearnerModelParam mparam{MakeMP(kCols, .5, 1)};
+
+  std::unique_ptr<GradientBooster> gbm{
+      CreateTrainedGBM("gblinear", Args{}, kRows, kCols, &mparam, &ctx)};
+  FeatureMap fmap;
+  ASSERT_THAT([&] { [[maybe_unused]] auto vec = gbm->DumpModel(fmap, true, "dot"); },
+              GMockThrow(R"(`dot` is not supported)"));
+}
+}  // namespace xgboost::gbm