Allow comparisons to refine Optional types without strict-optional

Fixes #4520.

Author: msullivan

mypy/checker.py

@@ -2862,7 +2862,7 @@ def find_isinstance_check(self, node: Expression
                 if literal(expr) == LITERAL_TYPE:
                     vartype = type_map[expr]
                     return self.conditional_callable_type_map(expr, vartype)
-        elif isinstance(node, ComparisonExpr) and experiments.STRICT_OPTIONAL:
+        elif isinstance(node, ComparisonExpr):
             # Check for `x is None` and `x is not None`.
             is_not = node.operators == ['is not']
             if any(is_literal_none(n) for n in node.operands) and (

test-data/unit/check-unions.test

@@ -947,3 +947,21 @@ MYTYPE = List[Union[str, "MYTYPE"]]
 [builtins fixtures/list.pyi]
 [out]
 main:2: error: Recursive types not fully supported yet, nested types replaced with "Any"
+
+[case testNonStrictOptional]
+from typing import Optional, List
+
+def union_test1(x):
+    # type: (Optional[List[int]]) -> Optional[int]
+    if x is None:
+        return x
+    else:
+        return x[0]
+
+def union_test2(x):
+    # type: (Optional[List[int]]) -> Optional[int]
+    if isinstance(x, type(None)):
+        return x
+    else:
+        return x[0]
+[builtins fixtures/isinstancelist.pyi]