Interpret bare ClassVar as inferred, not Any (#19573)

Fixes #5587

Apparently, it is part of _the spec_ now.

Author: ilevkivskyi

mypy/semanal.py

@@ -5094,6 +5094,7 @@ def check_classvar(self, s: AssignmentStmt) -> None:
             return
         if not s.type or not self.is_classvar(s.type):
             return
+        assert isinstance(s.type, UnboundType)
         if self.is_class_scope() and isinstance(lvalue, NameExpr):
             node = lvalue.node
             if isinstance(node, Var):
@@ -5110,6 +5111,12 @@ def check_classvar(self, s: AssignmentStmt) -> None:
             # In case of member access, report error only when assigning to self
             # Other kinds of member assignments should be already reported
             self.fail_invalid_classvar(lvalue)
+        if not s.type.args:
+            if isinstance(s.rvalue, TempNode) and s.rvalue.no_rhs:
+                if self.options.disallow_any_generics:
+                    self.fail("ClassVar without type argument becomes Any", s, code=codes.TYPE_ARG)
+                return
+            s.type = None
 
     def is_classvar(self, typ: Type) -> bool:
         if not isinstance(typ, UnboundType):

test-data/unit/check-classvar.test

@@ -360,3 +360,23 @@ reveal_type(C.x)  # E: Access to generic instance variables via class is ambiguo
                   # N: Revealed type is "Any"
 reveal_type(C.y)  # E: Access to generic class variables is ambiguous \
                   # N: Revealed type is "Any"
+
+[case testClassVarBareAnnotation]
+from typing import ClassVar
+
+class C:
+    x: ClassVar = 1
+    y: ClassVar
+
+reveal_type(C.x)  # N: Revealed type is "builtins.int"
+reveal_type(C().x)  # N: Revealed type is "builtins.int"
+reveal_type(C.y)  # N: Revealed type is "Any"
+reveal_type(C().y)  # N: Revealed type is "Any"
+
+[case testClassVarBareAnnotationDisabled]
+# flags: --disallow-any-generics
+from typing import ClassVar
+
+class C:
+    x: ClassVar = 1
+    y: ClassVar  # E: ClassVar without type argument becomes Any

test-data/unit/check-incremental.test

@@ -2051,7 +2051,7 @@ warn_no_return = True
 [case testIncrementalClassVar]
 from typing import ClassVar
 class A:
-    x = None  # type: ClassVar
+    x: ClassVar
 A().x = 0
 [out1]
 main:4: error: Cannot assign to class variable "x" via instance

test-data/unit/semanal-classvar.test

@@ -52,7 +52,7 @@ MypyFile:1(
     AssignmentStmt:3(
       NameExpr(x [m])
       IntExpr(1)
-      Any)))
+      builtins.int)))
 
 [case testClassVarWithTypeVar]