Report on too-convoluted interface/implementation hierarchies

src/mypy_zope/plugin.py

@@ -16,7 +16,7 @@
     ClassDefContext, SymbolTableNode
 )
 from mypy.semanal import SemanticAnalyzerPass2
-from mypy.mro import merge
+from mypy.mro import merge, MroError
 from mypy.options import Options
 
 from mypy.nodes import (
@@ -329,7 +329,13 @@ def _analyze_implementation(self, cls: ClassDef, iface_exprs: List[str],
             self.log(f"Adding {iface_expr} to MRO of {info.fullname()}")
             seqs.append(cast(TypeInfo, stn.node).mro)
 
-        info.mro = merge(seqs)
+        try:
+            info.mro = merge(seqs)
+        except MroError:
+            hierarchies = [[i.fullname() for i in s] for s in seqs]
+            api.fail(f"Unable to calculate a consistent MRO: cannot merge class hierarchies:", info)
+            for h in hierarchies:
+                api.fail(f"  -> {h}", info)
 
         # XXX: Reuse abstract status checker from SemanticAnalyzerPass2.
         # Ideally, implement a dedicated interface verifier.

tests/samples/bad_mro.py

@@ -0,0 +1,36 @@
+"""It is not always possible to consistently inject complicated
+interface hierarchy into implementation MRO. We warn about such occurances.
+
+By itself, both interface hierarchy and implementation hierarchies are
+consistent, but combining them is impossible - resulting inheritance graph
+become unsortable.
+"""
+import zope.interface
+
+class ICreature(zope.interface.Interface): pass
+class ISwimmable(zope.interface.Interface): pass
+class IReptilia(ICreature): pass
+class ICrocodile(ICreature, ISwimmable): pass
+
+@zope.interface.implementer(ISwimmable)
+class Swimmable: pass
+@zope.interface.implementer(IReptilia)
+class Reptilia(Swimmable): pass
+@zope.interface.implementer(ICrocodile)
+class Crocodile(Reptilia): pass
+
+
+def main() -> None:
+    croc = Crocodile()
+    print(croc)
+
+if __name__ == '__main__':
+    main()
+
+"""
+<output>
+bad_mro.py:19: error: Unable to calculate a consistent MRO: cannot merge class hierarchies:
+bad_mro.py:19: error:   -> ['__main__.Crocodile', '__main__.Reptilia', '__main__.Swimmable', '__main__.ISwimmable', '__main__.IReptilia', '__main__.ICreature', 'zope.interface.Interface', 'builtins.object']
+bad_mro.py:19: error:   -> ['__main__.ICrocodile', '__main__.ICreature', '__main__.ISwimmable', 'zope.interface.Interface', 'builtins.object']
+</output>
+"""