[mypyc] Reject instance attribute access through class object (#10798)

Accessing an instance attribute through a native class object results in
unexpected behavior at runtime (e.g. <attribute 'x' of 'C' objects>)
so reject these during compilation.

Also produce a note that suggests how to work around the issue.

Fixes mypyc/mypyc#814.

mypyc/irbuild/expression.py

@@ -14,7 +14,7 @@
     AssignmentExpr,
     Var, RefExpr, MypyFile, TypeInfo, TypeApplication, LDEF, ARG_POS
 )
-from mypy.types import TupleType, get_proper_type, Instance
+from mypy.types import TupleType, Instance, TypeType, ProperType, get_proper_type
 
 from mypyc.common import MAX_SHORT_INT
 from mypyc.ir.ops import (
@@ -133,9 +133,41 @@ def transform_member_expr(builder: IRBuilder, expr: MemberExpr) -> Value:
         if expr.name in fields:
             index = builder.builder.load_int(fields.index(expr.name))
             return builder.gen_method_call(obj, '__getitem__', [index], rtype, expr.line)
+
+    check_instance_attribute_access_through_class(builder, expr, typ)
+
     return builder.builder.get_attr(obj, expr.name, rtype, expr.line)
 
 
+def check_instance_attribute_access_through_class(builder: IRBuilder,
+                                                  expr: MemberExpr,
+                                                  typ: Optional[ProperType]) -> None:
+    """Report error if accessing an instance attribute through class object."""
+    if isinstance(expr.expr, RefExpr):
+        node = expr.expr.node
+        if isinstance(typ, TypeType) and isinstance(typ.item, Instance):
+            # TODO: Handle other item types
+            node = typ.item.type
+        if isinstance(node, TypeInfo):
+            class_ir = builder.mapper.type_to_ir.get(node)
+            if class_ir is not None and class_ir.is_ext_class:
+                sym = node.get(expr.name)
+                if (sym is not None
+                        and isinstance(sym.node, Var)
+                        and not sym.node.is_classvar
+                        and not sym.node.is_final):
+                    builder.error(
+                        'Cannot access instance attribute "{}" through class object'.format(
+                            expr.name),
+                        expr.line
+                    )
+                    builder.note(
+                        '(Hint: Use "x: Final = ..." or "x: ClassVar = ..." to define '
+                        'a class attribute)',
+                        expr.line
+                    )
+
+
 def transform_super_expr(builder: IRBuilder, o: SuperExpr) -> Value:
     # warning(builder, 'can not optimize super() expression', o.line)
     sup_val = builder.load_module_attr_by_fullname('builtins.super', o.line)

mypyc/test-data/irbuild-classes.test

@@ -1144,3 +1144,41 @@ class DeletableFinal2:
     X: Final = 0  # E: Deletable attribute cannot be final
 
     __deletable__ = ['X']
+
+[case testNeedAnnotateClassVar]
+from typing import Final, ClassVar, Type
+
+class C:
+    a = 'A'
+    b: str = 'B'
+    f: Final = 'F'
+    c: ClassVar = 'C'
+
+class D(C):
+    pass
+
+def f() -> None:
+    C.a  # E: Cannot access instance attribute "a" through class object \
+         # N: (Hint: Use "x: Final = ..." or "x: ClassVar = ..." to define a class attribute)
+    C.b  # E: Cannot access instance attribute "b" through class object \
+         # N: (Hint: Use "x: Final = ..." or "x: ClassVar = ..." to define a class attribute)
+    C.f
+    C.c
+
+    D.a  # E: Cannot access instance attribute "a" through class object \
+         # N: (Hint: Use "x: Final = ..." or "x: ClassVar = ..." to define a class attribute)
+    D.b  # E: Cannot access instance attribute "b" through class object \
+         # N: (Hint: Use "x: Final = ..." or "x: ClassVar = ..." to define a class attribute)
+    D.f
+    D.c
+
+def g(c: Type[C], d: Type[D]) -> None:
+    c.a  # E: Cannot access instance attribute "a" through class object \
+         # N: (Hint: Use "x: Final = ..." or "x: ClassVar = ..." to define a class attribute)
+    c.f
+    c.c
+
+    d.a  # E: Cannot access instance attribute "a" through class object \
+         # N: (Hint: Use "x: Final = ..." or "x: ClassVar = ..." to define a class attribute)
+    d.f
+    d.c

mypyc/test-data/run-classes.test

@@ -225,8 +225,15 @@ class Overload:
 def get(c: Overload, s: str) -> str:
     return c.get(s)
 
+@decorator
+class Var:
+    x = 'xy'
+
+def get_class_var() -> str:
+    return Var.x
+
 [file driver.py]
-from native import A, Overload, get
+from native import A, Overload, get, get_class_var
 a = A()
 assert a.a == 1
 assert a.b == 2
@@ -240,6 +247,8 @@ o = Overload()
 assert get(o, "test") == "test"
 assert o.get(20) == 20
 
+assert get_class_var() == 'xy'
+
 [case testEnum]
 from enum import Enum