Fix ClassVar generation for msgspec Struct discriminator without use_annotated (#2625)

Author: koxudaxi

src/datamodel_code_generator/model/msgspec.py

@@ -317,40 +317,44 @@ def _get_meta_string(self) -> str | None:
 
     @property
     def annotated(self) -> str | None:
-        """Get Annotated type hint with Meta constraints."""
+        """Get Annotated type hint with Meta constraints.
+
+        For ClassVar fields (discriminator tag_field), ClassVar is required
+        regardless of use_annotated setting.
+        """
+        if self.extras.get("is_classvar"):
+            meta = self._get_meta_string()
+            if self.use_annotated and meta:
+                annotated_type = f"Annotated[{self.type_hint}, {meta}]"
+                return f"ClassVar[{annotated_type}]"
+            return f"ClassVar[{self.type_hint}]"
+
         if not self.use_annotated:  # pragma: no cover
             return None
 
         meta = self._get_meta_string()
 
-        if not meta and not self.extras.get("is_classvar"):
+        if not meta:
             return None
 
-        if not self.required and not self.extras.get("is_classvar"):
+        if not self.required:
             type_hint = self.data_type.type_hint
             annotated_type = f"Annotated[{type_hint}, {meta}]"
             return get_neither_required_nor_nullable_type(annotated_type, self.data_type.use_union_operator)
 
-        # Handle ClassVar case (for discriminator fields in msgspec)
-        if self.extras.get("is_classvar"):
-            if meta:
-                annotated_type = f"Annotated[{self.type_hint}, {meta}]"
-                return f"ClassVar[{annotated_type}]"
-            return f"ClassVar[{self.type_hint}]"
-
         return f"Annotated[{self.type_hint}, {meta}]"
 
     @property
     def needs_annotated_import(self) -> bool:
         """Check if this field requires the Annotated import.
 
-        ClassVar fields without Meta constraints don't need Annotated.
+        ClassVar fields with Meta need Annotated only when use_annotated is True.
+        ClassVar fields without Meta don't need Annotated.
         """
         if not self.annotated:
             return False
-        # ClassVar without Meta doesn't use Annotated
         if self.extras.get("is_classvar"):
-            return self._get_meta_string() is not None
+            return self.use_annotated and self._get_meta_string() is not None
         return True
 
     @property

tests/data/expected/main/jsonschema/discriminator_with_type_string_msgspec_no_annotated.py

@@ -0,0 +1,23 @@
+# generated by datamodel-codegen:
+#   filename:  discriminator_with_type_string.json
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import ClassVar, Literal, Union
+
+from msgspec import UNSET, Struct, UnsetType
+
+
+class SystemMessage(Struct, tag_field='role', tag='system'):
+    role: ClassVar[Literal['system']]
+    content: str
+
+
+class UserMessage(Struct, tag_field='role', tag='user'):
+    role: ClassVar[Literal['user']]
+    content: str
+
+
+class Model(Struct):
+    message: Union[SystemMessage, UserMessage, UnsetType] = UNSET

tests/main/jsonschema/test_main_jsonschema.py

@@ -2521,6 +2521,19 @@ def test_main_msgspec_discriminator_with_meta(output_file: Path) -> None:
     )
 
 
+@MSGSPEC_LEGACY_BLACK_SKIP
+def test_main_msgspec_discriminator_without_annotated(output_file: Path) -> None:
+    """Test msgspec Struct discriminator generates ClassVar even without use_annotated."""
+    generate(
+        JSON_SCHEMA_DATA_PATH / "discriminator_with_type_string.json",
+        output=output_file,
+        output_model_type=DataModelType.MsgspecStruct,
+        target_python_version=PythonVersion.PY_310,
+        use_annotated=False,
+    )
+    assert_file_content(output_file, "discriminator_with_type_string_msgspec_no_annotated.py")
+
+
 @MSGSPEC_LEGACY_BLACK_SKIP
 def test_main_msgspec_null_field(output_file: Path) -> None:
     """Test msgspec Struct generation with null type fields."""