Inferred subclass override of a ClassVar is also a ClassVar (#4718)

Fixes #4715.

Author: carljm

mypy/semanal.py

@@ -1832,6 +1832,7 @@ def analyze_lvalue(self, lval: Lvalue, nested: bool = False,
                 v = Var(lval.name)
                 v.info = self.type
                 v.is_initialized_in_class = True
+                v.is_inferred = not explicit_type
                 v.set_line(lval)
                 v._fullname = self.qualified_name(lval.name)
                 lval.node = v

mypy/semanal_pass3.py

@@ -18,7 +18,8 @@
     Node, Expression, MypyFile, FuncDef, FuncItem, Decorator, RefExpr, Context, TypeInfo, ClassDef,
     Block, TypedDictExpr, NamedTupleExpr, AssignmentStmt, IndexExpr, TypeAliasExpr, NameExpr,
     CallExpr, NewTypeExpr, ForStmt, WithStmt, CastExpr, TypeVarExpr, TypeApplication, Lvalue,
-    TupleExpr, RevealTypeExpr, SymbolTableNode, SymbolTable, Var, ARG_POS, OverloadedFuncDef
+    TupleExpr, RevealTypeExpr, SymbolTableNode, SymbolTable, Var, ARG_POS, OverloadedFuncDef,
+    MDEF,
 )
 from mypy.types import (
     Type, Instance, AnyType, TypeOfAny, CallableType, TupleType, TypeVarType, TypedDictType,
@@ -253,6 +254,18 @@ def visit_assignment_stmt(self, s: AssignmentStmt) -> None:
                 node = self.sem.lookup(s.lvalues[0].name, s, suppress_errors=True)
                 if node:
                     self.analyze(node.type_override, node)
+        # Subclass attribute assignments with no type annotation should be
+        # assumed to be classvar if overriding a declared classvar from the base
+        # class.
+        if (isinstance(s.lvalues[0], NameExpr) and s.lvalues[0].kind == MDEF
+                and isinstance(s.lvalues[0].node, Var)):
+            var = s.lvalues[0].node
+            if var.info is not None and var.is_inferred and not var.is_classvar:
+                for base in var.info.mro[1:]:
+                    tnode = base.names.get(var.name())
+                    if (tnode is not None and isinstance(tnode.node, Var)
+                            and tnode.node.is_classvar):
+                        var.is_classvar = True
         super().visit_assignment_stmt(s)
 
     def visit_for_stmt(self, s: ForStmt) -> None:

test-data/unit/check-classvar.test

@@ -245,6 +245,22 @@ class A:
 class B(A):
     x = 2  # type: ClassVar[int]
 
+[case testOverrideClassVarWithImplicitClassVar]
+from typing import ClassVar
+class A:
+    x = 1  # type: ClassVar[int]
+class B(A):
+    x = 2
+
+[case testOverrideClassVarWithImplicitThenExplicit]
+from typing import ClassVar
+class A:
+    x = 1  # type: ClassVar[int]
+class B(A):
+    x = 2
+class C(B):
+    x = 3  # type: ClassVar[int]
+
 [case testOverrideOnABCSubclass]
 from abc import ABCMeta
 from typing import ClassVar