fix: validate include-group items as strings and accept Mapping items

CHANGELOG.rst

@@ -8,6 +8,9 @@ Unreleased
 - Remove support for Python 3.8
 - Correct the type of the ``dependency_groups`` parameter to ``resolve()``,
   ``resolve_all()``, and ``DependencyGroupsResolver()``.
+- Raise a clear ``TypeError`` when an ``include-group`` value is not a string
+  (was a cryptic ``TypeError`` from name normalization), and accept any
+  ``Mapping`` for include items, not only ``dict``.
 
 1.3.1
 -----

src/dependency_groups/_implementation.py

@@ -154,11 +154,15 @@ def _parse_group(
                 # valid PEP 508 Dependency Specifier
                 # raises InvalidRequirement on failure
                 elements.append(Requirement(item))
-            elif isinstance(item, dict):
+            elif isinstance(item, Mapping):
                 if tuple(item.keys()) != ("include-group",):
                     raise ValueError(f"Invalid dependency group item: {item}")
 
                 include_group = next(iter(item.values()))
+                if not isinstance(include_group, str):
+                    raise TypeError(
+                        f"Invalid include-group value, must be a string: {item}"
+                    )
                 elements.append(DependencyGroupInclude(include_group=include_group))
             else:
                 raise ValueError(f"Invalid dependency group item: {item}")

tests/test_resolve_func.py

@@ -166,6 +166,27 @@ def test_unknown_object_shape(item):
         resolve(groups, "test")
 
 
+def test_non_str_include_group_value():
+    groups = {"test": [{"include-group": 5}]}
+    with pytest.raises(
+        TypeError, match="Invalid include-group value, must be a string:"
+    ):
+        resolve(groups, "test")
+
+
+def test_mapping_include_group_item():
+    import types
+
+    groups = {
+        "test": [
+            "pytest",
+            types.MappingProxyType({"include-group": "runtime"}),
+        ],
+        "runtime": ["sqlalchemy"],
+    }
+    assert set(resolve(groups, "test")) == {"pytest", "sqlalchemy"}
+
+
 def test_resolve_all_empty():
     groups = {}
     assert resolve_all(groups) == {}