Test that illustrates sympy metaclass problem

Author: sobolevn

test-data/unit/check-classes.test

@@ -4351,7 +4351,7 @@ class C(B):
 class X(type): pass
 class Y(type): pass
 class A(metaclass=X): pass
-class B(A, metaclass=Y): pass  # E: Inconsistent metaclass structure for "B"
+class B(A, metaclass=Y): pass  # E: Metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases
 
 [case testMetaclassNoTypeReveal]
 class M:
@@ -5213,8 +5213,8 @@ class CD(six.with_metaclass(M)): pass  # E: Multiple metaclass definitions
 class M1(type): pass
 class Q1(metaclass=M1): pass
 @six.add_metaclass(M)
-class CQA(Q1): pass  # E: Inconsistent metaclass structure for "CQA"
-class CQW(six.with_metaclass(M, Q1)): pass  # E: Inconsistent metaclass structure for "CQW"
+class CQA(Q1): pass  # E: Metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases
+class CQW(six.with_metaclass(M, Q1)): pass  # E: Metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases
 [builtins fixtures/tuple.pyi]
 
 [case testSixMetaclassAny]
@@ -5319,7 +5319,7 @@ class C5(future.utils.with_metaclass(f())): pass  # E: Dynamic metaclass not sup
 
 class M1(type): pass
 class Q1(metaclass=M1): pass
-class CQW(future.utils.with_metaclass(M, Q1)): pass  # E: Inconsistent metaclass structure for "CQW"
+class CQW(future.utils.with_metaclass(M, Q1)): pass  # E: Metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases
 [builtins fixtures/tuple.pyi]
 
 [case testFutureMetaclassAny]
@@ -6663,6 +6663,39 @@ class MyMetaClass(type):
 class MyClass(metaclass=MyMetaClass):
     pass
 
+
+[case testMetaclassPlaceholderNode]
+from sympy.assumptions import ManagedProperties
+from sympy.ops import AssocOp
+reveal_type(AssocOp.x)  # N: Revealed type is "sympy.basic.Basic"
+reveal_type(AssocOp.y)  # N: Revealed type is "builtins.int"
+
+[file sympy/__init__.py]
+
+[file sympy/assumptions.py]
+from .basic import Basic
+class ManagedProperties(type):
+    x: Basic
+    y: int
+# The problem is with the next line,
+# it creates the following order (classname, metaclass):
+# 1. Basic NameExpr(ManagedProperties)
+# 2. AssocOp None
+# 3. ManagedProperties None
+# 4. Basic NameExpr(ManagedProperties [sympy.assumptions.ManagedProperties])
+# So, `AssocOp` will still have `metaclass_type` as `None`
+# and all its `mro` types will have `declared_metaclass` as `None`.
+from sympy.ops import AssocOp
+
+[file sympy/basic.py]
+from .assumptions import ManagedProperties
+class Basic(metaclass=ManagedProperties): ...
+
+[file sympy/ops.py]
+from sympy.basic import Basic
+class AssocOp(Basic): ...
+
+
 [case testGenericOverride]
 from typing import Generic, TypeVar, Any