Fix type alias template crash with reuse_model on empty fields (#3060)

* Fix type alias template crash when reuse_model creates empty fields

Guard fields[0] access in TypeStatement, TypeAliasType, TypeAliasAnnotation,
and UnionTypeStatement templates. When fields is empty (reuse model), fall
back to base_class, producing e.g. `type Foo = Bar`.

Fixes #3059

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Fix same empty fields crash in UnionTypeAliasAnnotation and UnionTypeAliasType

These two union templates had the identical unguarded fields[0] access.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Skip reuse_model type alias test on old black versions

Black 22/23 can't parse Python 3.14 type statement syntax.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Rename input fixture to match expected output filename

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: butvinm

src/datamodel_code_generator/model/template/TypeAliasAnnotation.jinja2

@@ -8,13 +8,22 @@ Annotated[{{ _field.type_hint }}, {{ _field.field }}]
 {%- endif -%}
 {%- endmacro -%}
 
+{%- if fields %}
 {{ class_name }}: TypeAlias = {{ get_type_annotation(fields[0]) }}{% if comment is defined %}  # {{ comment }}{% endif %}
 {%- if description %}
 """
 {{ description | escape_docstring | indent(0) }}
 """
-{%- elif fields and fields[0].docstring %}
+{%- elif fields[0].docstring %}
 """
 {{ fields[0].docstring | escape_docstring | indent(0) }}
 """
 {%- endif %}
+{%- else %}
+{{ class_name }}: TypeAlias = {{ base_class }}{% if comment is defined %}  # {{ comment }}{% endif %}
+{%- if description %}
+"""
+{{ description | escape_docstring | indent(0) }}
+"""
+{%- endif %}
+{%- endif %}

src/datamodel_code_generator/model/template/TypeAliasType.jinja2

@@ -8,13 +8,22 @@ Annotated[{{ _field.type_hint }}, {{ _field.field }}]
 {%- endif -%}
 {%- endmacro -%}
 
+{%- if fields %}
 {{ class_name }} = TypeAliasType("{{ class_name }}", {{ get_type_annotation(fields[0]) }}){% if comment is defined %}  # {{ comment }}{% endif %}
 {%- if description %}
 """
 {{ description | escape_docstring | indent(0) }}
 """
-{%- elif fields and fields[0].docstring %}
+{%- elif fields[0].docstring %}
 """
 {{ fields[0].docstring | escape_docstring | indent(0) }}
 """
 {%- endif %}
+{%- else %}
+{{ class_name }} = TypeAliasType("{{ class_name }}", {{ base_class }}){% if comment is defined %}  # {{ comment }}{% endif %}
+{%- if description %}
+"""
+{{ description | escape_docstring | indent(0) }}
+"""
+{%- endif %}
+{%- endif %}

src/datamodel_code_generator/model/template/TypeStatement.jinja2

@@ -8,13 +8,22 @@ Annotated[{{ _field.type_hint }}, {{ _field.field }}]
 {%- endif -%}
 {%- endmacro -%}
 
+{%- if fields %}
 type {{ class_name }} = {{ get_type_annotation(fields[0]) }}{% if comment is defined %}  # {{ comment }}{% endif %}
 {%- if description %}
 """
 {{ description | escape_docstring | indent(0) }}
 """
-{%- elif fields and fields[0].docstring %}
+{%- elif fields[0].docstring %}
 """
 {{ fields[0].docstring | escape_docstring | indent(0) }}
 """
+{%- endif %}
+{%- else %}
+type {{ class_name }} = {{ base_class }}{% if comment is defined %}  # {{ comment }}{% endif %}
+{%- if description %}
+"""
+{{ description | escape_docstring | indent(0) }}
+"""
+{%- endif %}
 {%- endif %}

src/datamodel_code_generator/model/template/UnionTypeAliasAnnotation.jinja2

@@ -6,5 +6,6 @@
 {%- for field in fields %}
         '{{ field.name }}',
 {%- endfor %}
-    ]{% else %}
-{{ class_name }}: TypeAlias = {{ fields[0].name }}{% endif %}
+    ]{% elif fields %}
+{{ class_name }}: TypeAlias = {{ fields[0].name }}{% else %}
+{{ class_name }}: TypeAlias = {{ base_class }}{% endif %}

src/datamodel_code_generator/model/template/UnionTypeAliasType.jinja2

@@ -6,5 +6,6 @@
 {%- for field in fields %}
     '{{ field.name }}',
 {%- endfor %}
-]){% else %}
-{{ class_name }} = TypeAliasType("{{ class_name }}", {{ fields[0].name }}){% endif %}
+]){% elif fields %}
+{{ class_name }} = TypeAliasType("{{ class_name }}", {{ fields[0].name }}){% else %}
+{{ class_name }} = TypeAliasType("{{ class_name }}", {{ base_class }}){% endif %}

src/datamodel_code_generator/model/template/UnionTypeStatement.jinja2

@@ -6,5 +6,6 @@ type {{ class_name }} = Union[
 {%- for field in fields %}
     '{{ field.name }}',
 {%- endfor %}
-]{% else %}
-type {{ class_name }} = {{ fields[0].name }}{% endif %}
+]{% elif fields %}
+type {{ class_name }} = {{ fields[0].name }}{% else %}
+type {{ class_name }} = {{ base_class }}{% endif %}

tests/data/expected/main/openapi/reuse_model_with_type_alias.py

@@ -0,0 +1,8 @@
+# generated by datamodel-codegen:
+#   filename:  reuse_model_with_type_alias.json
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+type Foo = str
+
+
+type Bar = Foo

tests/data/openapi/reuse_model_with_type_alias.json

@@ -0,0 +1,11 @@
+{
+  "openapi": "3.0.0",
+  "info": {"title": "Test", "version": "1.0.0"},
+  "paths": {},
+  "components": {
+    "schemas": {
+      "Foo": {"type": "string"},
+      "Bar": {"type": "string"}
+    }
+  }
+}

tests/main/openapi/test_main_openapi.py

@@ -5224,3 +5224,25 @@ def test_ref_merge_parameters(output_file: Path) -> None:
             "parameters",
         ],
     )
+
+
+@BLACK_PY314_SKIP
+def test_main_reuse_model_with_type_alias(output_file: Path) -> None:
+    """Test --reuse-model with --use-type-alias doesn't crash on empty fields.
+
+    Regression test for https://github.com/koxudaxi/datamodel-code-generator/issues/3059
+    """
+    run_main_and_assert(
+        input_path=OPEN_API_DATA_PATH / "reuse_model_with_type_alias.json",
+        output_path=output_file,
+        input_file_type="openapi",
+        assert_func=assert_file_content,
+        extra_args=[
+            "--output-model-type",
+            "pydantic_v2.BaseModel",
+            "--target-python-version",
+            "3.14",
+            "--reuse-model",
+            "--use-type-alias",
+        ],
+    )