Ban diamond inheritance of generic base classes

Fixes python#1065.

mypy/maptype.py

@@ -50,6 +50,8 @@ def class_derivation_paths(typ: TypeInfo,
     """
     # FIX: Currently we might only ever have a single path, so this could be
     #      simplified
+    # TODO(gregprice): Not actually true!  But all paths are equivalent for
+    # this module's purposes.
     result = []  # type: List[List[TypeInfo]]
 
     for base in typ.bases:

mypy/semanal.py

@@ -731,6 +731,9 @@ def verify_base_classes(self, defn: ClassDef) -> bool:
                 info.bases = []
                 return False # ???
             for base in nxt.bases:
+                if base.type in visited and len(base.type.type_vars) > 0:
+                    self.fail('Diamond inheritance of generic base class "%s"'
+                              % base.type.name(), defn)
                 if base.type not in visited:
                     worklist.append(base.type)
                     visited.add(base.type)

mypy/test/data/check-multiple-inheritance.test

@@ -219,6 +219,33 @@ main: note: In class "D":
 main:8: error: Definition of "clear" in base class "A" is incompatible with definition in base class "B"
 
 
+-- Diamond inheritance hierarchies
+-- -------------------------------
+
+
+[case testDiamondPlain]
+class A: pass
+class B(A): pass
+class C(A): pass
+class D(B, C): pass
+
+a = None # type: A
+d = None # type: D
+a = d
+
+[case testDiamondGeneric]
+from typing import TypeVar, Generic
+T = TypeVar('T')
+class A(Generic[T]): pass
+class B(Generic[T], A[T]): pass
+class C(Generic[T], A[T]): pass
+class D(Generic[T], B[T], C[T]): pass # E: Diamond inheritance of generic base class "A"
+
+a = None # type: A[int]
+d = None # type: D[int]
+a = d
+
+
 -- Special cases
 -- -------------