Add false assertion, and diamond-inheritance test case, to test comment

mypy/maptype.py

@@ -74,6 +74,7 @@ def class_derivation_paths(typ: TypeInfo,
     """
     # FIX: Currently we might only ever have a single path, so this could be
     #      simplified
+    # (But see assert below!  It can fail.)
     result = []  # type: List[List[TypeInfo]]
 
     for base in typ.bases:
@@ -84,6 +85,8 @@ def class_derivation_paths(typ: TypeInfo,
             for path in class_derivation_paths(base.type, supertype):
                 result.append([base.type] + path)
 
+    assert len(result) == 1, str(result)
+
     return result
 
 

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

@@ -219,6 +219,23 @@ 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 testDiamond]
+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
+
+a = None # type: A[int]
+d = None # type: D[int]
+a = d
+
+
 -- Special cases
 -- -------------