fix: write -inf split conditions as -Infinity in json dumps

src/tree/tree_model.cc

@@ -72,6 +72,16 @@ std::string ToStr(linalg::VectorView<Float> value, bst_target_t truncate_limit =
   ss << value(value.Size() - 1) << "]";
   return ss.str();
 }
+
+std::string ToJsonStr(float value) {
+  if (std::isfinite(value)) {
+    return ToStr(value);
+  }
+  // The stream writes `inf` and `nan`, which are not JSON. Write them as the model JSON does.
+  std::string str;
+  Json::Dump(Json{JsonNumber{value}}, &str);
+  return str;
+}
 }  // namespace
 
 /**
@@ -406,6 +416,9 @@ class JsonGenerator : public TreeGenerator<TreeView> {
 
   std::string Integer(TreeView tree, int32_t nid, uint32_t depth) const override {
     auto cond = tree.SplitCond(nid);
+    if (!std::isfinite(cond)) {
+      return this->Quantitive(tree, nid, depth);  // No integer form, write it as a float.
+    }
     const bst_float floored = std::floor(cond);
     const int32_t integer_threshold =
         (floored == cond) ? static_cast<int32_t>(floored) : static_cast<int32_t>(floored) + 1;
@@ -422,7 +435,7 @@ class JsonGenerator : public TreeGenerator<TreeView> {
         R"I("split_condition": {cond}, "yes": {left}, "no": {right}, )I"
         R"I("missing": {missing})I";
     bst_float cond = tree.SplitCond(nid);
-    return SplitNodeImpl(tree, nid, kQuantitiveTemplate, ToStr(cond), depth);
+    return SplitNodeImpl(tree, nid, kQuantitiveTemplate, ToJsonStr(cond), depth);
   }
 
   std::string PlainNode(TreeView tree, int32_t nid, uint32_t depth) const override {
@@ -431,7 +444,7 @@ class JsonGenerator : public TreeGenerator<TreeView> {
         R"I( "nodeid": {nid}, "depth": {depth}, "split": "{fname}", )I"
         R"I("split_condition": {cond}, "yes": {left}, "no": {right}, )I"
         R"I("missing": {missing})I";
-    return SplitNodeImpl(tree, nid, kNodeTemplate, ToStr(cond), depth);
+    return SplitNodeImpl(tree, nid, kNodeTemplate, ToJsonStr(cond), depth);
   }
 
   std::string NodeStat(TreeView tree, int32_t nid) const override {

tests/cpp/tree/test_tree_model.cc

@@ -306,6 +306,24 @@ TEST(Tree, DumpText) {
 
 TEST(Tree, DumpTextCategorical) { TestCategoricalTreeDump("text", ","); }
 
+TEST(Tree, DumpNonFiniteSplit) {
+  // GPU hist splits at the `-inf` lower bound of the first bin, sending every value right.
+  auto constexpr kNegInf = -std::numeric_limits<float>::infinity();
+  RegTree tree;
+  tree.Expand({{0, 0, kNegInf, true}, {0.0f, 0.0f}, {0.0f, 0.0f}, {0.0f, 0.0f}, 0.0f});
+  tree.FinalizeLeaves(1.0f);
+
+  FeatureMap fmap;
+  auto str = tree.DumpModel(fmap, false, "json");
+  auto j_tree = Json::Load({str.c_str(), str.size()});
+  ASSERT_EQ(get<Number const>(j_tree["split_condition"]), kNegInf);
+
+  fmap.PushBack(0, "feat_0", "int");
+  str = tree.DumpModel(fmap, false, "json");
+  j_tree = Json::Load({str.c_str(), str.size()});
+  ASSERT_EQ(get<Number const>(j_tree["split_condition"]), kNegInf);
+}
+
 TEST(Tree, DumpDot) {
   auto tree = ConstructTree();
   FeatureMap fmap;