Check right operand of is expressions (#4243)

The right operand of `is` expressions were not being checked.
Fixes #4055

mypy/checkexpr.py

@@ -1395,7 +1395,7 @@ def visit_comparison_expr(self, e: ComparisonExpr) -> Type:
             method_type = None  # type: Optional[mypy.types.Type]
 
             if operator == 'in' or operator == 'not in':
-                right_type = self.accept(right)  # TODO only evaluate if needed
+                right_type = self.accept(right)  # always validate the right operand
 
                 # Keep track of whether we get type check errors (these won't be reported, they
                 # are just to verify whether something is valid typing wise).
@@ -1429,6 +1429,7 @@ def visit_comparison_expr(self, e: ComparisonExpr) -> Type:
                                                     allow_reverse=True)
 
             elif operator == 'is' or operator == 'is not':
+                self.accept(right)  # validate the right operand
                 sub_result = self.bool_type()
                 method_type = None
             else:

test-data/unit/check-expressions.test

@@ -613,6 +613,13 @@ class A: pass
 [out]
 main:3: error: Incompatible types in assignment (expression has type "bool", variable has type "A")
 
+[case testIsRightOperand]
+
+1 is 1()
+[builtins fixtures/bool.pyi]
+[out]
+main:2: error: "int" not callable
+
 [case testReverseBinaryOperator]
 
 class A: