cpp/apigen: Fix handling of unnamed template parameters

Previously, unnamed template parameters were incorrectly assigned a name
of `"None"`.

sphinx_immaterial/apidoc/cpp/api_parser.py

@@ -1004,10 +1004,11 @@ def get_nonitpick_directives(decl: Cursor) -> List[str]:
 def _clang_template_parameter_to_json(config: Config, decl: Cursor):
     param_decl_str = get_extent_spelling(decl.translation_unit, decl.extent)
     param = _parse_template_parameter(param_decl_str)
+    spelling = decl.spelling
     if param is None:
         return {
             "declaration": param_decl_str,
-            "name": decl.spelling,
+            "name": spelling if spelling else None,
             "kind": TEMPLATE_PARAMETER_KIND_TO_JSON_KIND[decl.kind],
             # Heuristic to determine if it is a pack.
             "pack": "..." in param_decl_str,
@@ -1506,9 +1507,11 @@ def _sphinx_ast_template_parameter_to_json(
     else:
         kind = "non_type"
 
+    identifier = param.get_identifier()
+
     return {
         "declaration": _substitute_internal_type_names(config, str(param)),
-        "name": str(param.get_identifier()),
+        "name": str(identifier) if identifier else None,
         "kind": cast(TemplateParameterKind, kind),
         "pack": param.isPack,  # type: ignore[attr-defined]
     }
@@ -1941,7 +1944,10 @@ def _transform_template_parameters(config: Config, template_parameters: List[Any
         # the substitution `{"SfinaeXXX": "Condition"}`.  To avoid parsing in cases
         # that can't possibly match, first look to see if the name starts with
         # `"sfinae"`.
-        if re.match(r"sfinae", template_parameter["name"], re.IGNORECASE) is not None:
+        if (
+            re.match(r"sfinae", template_parameter["name"] or "", re.IGNORECASE)
+            is not None
+        ):
             # Possibly match, parse to verify.
             replacement = _extract_sfinae_replacement(decl)
             if replacement is not None:

tests/cpp_api_parser_test.py

@@ -222,3 +222,23 @@ def test_function_fields():
         ]
     )
     assert expected == doc_str
+
+
+def test_unnamed_template_parameter():
+    config = api_parser.Config(
+        input_path="a.cpp",
+        compiler_flags=["-std=c++17", "-x", "c++"],
+        input_content=rb"""
+/// Tests something.
+///
+/// \ingroup Array
+template <typename = void>
+constexpr inline bool IsArray = false;
+""",
+    )
+
+    output = api_parser.generate_output(config)
+    entities = output.get("entities", {})
+    assert len(entities) == 1
+    entity = list(entities.values())[0]
+    assert entity["template_parameters"][0]["name"] is None

tests/cpp_apigen_test.py

@@ -0,0 +1,38 @@
+"""Tests cpp/apigen."""
+
+
+def test_unnamed_template_parameter(immaterial_make_app):
+    app = immaterial_make_app(
+        extra_conf="""
+extensions.append("sphinx_immaterial.apidoc.cpp.apigen")
+""",
+        confoverrides=dict(
+            nitpicky=True,
+            cpp_apigen_configs=[
+                dict(
+                    document_prefix="cpp_apigen_generated/",
+                    api_parser_config=dict(
+                        input_content=r"""
+/// Tests if something is an array.
+///
+/// \ingroup Array
+template <typename T, typename = void>
+constexpr inline bool IsArray = false;
+""",
+                        compiler_flags=["-std=c++17", "-x", "c++"],
+                        verbose=True,
+                    ),
+                ),
+            ],
+        ),
+        files={
+            "index.rst": """
+.. cpp-apigen-group:: Array
+
+"""
+        },
+    )
+
+    app.build()
+
+    assert not app._warning.getvalue()