Support use of TypeVar on generic subclasses (#842)

* Add failing generic subclass test

* Only raise type parameter failure on base GenericModel class

* Add changes to describe PR #842

* Change the class check to use is

* Fix formatting in subclass test

* correct change

changes/842-zpencerq.rst

@@ -0,0 +1 @@
+Only check ``TypeVar`` param on base ``GenericModel`` class

pydantic/generics.py

@@ -27,7 +27,7 @@ class GenericModel(BaseModel):
             raise TypeError('Cannot parameterize a concrete instantiation of a generic model')
         if not isinstance(params, tuple):
             params = (params,)
-        if any(isinstance(param, TypeVar) for param in params):  # type: ignore
+        if cls is GenericModel and any(isinstance(param, TypeVar) for param in params):  # type: ignore
             raise TypeError(f'Type parameters should be placed on typing.Generic, not GenericModel')
         if Generic not in cls.__bases__:
             raise TypeError(f'Type {cls.__name__} must inherit from typing.Generic before being parameterized')

tests/test_generics.py

@@ -174,6 +174,16 @@ def test_parameters_must_be_typevar():
     assert str(exc_info.value) == f'Type parameters should be placed on typing.Generic, not GenericModel'
 
 
+@skip_36
+def test_subclass_can_be_genericized():
+    T = TypeVar('T')
+
+    class Result(GenericModel, Generic[T]):
+        pass
+
+    Result[T]
+
+
 @skip_36
 def test_parameter_count():
     T = TypeVar('T')