Suppress any errors found in a boolean expression after filtering out all possible types.

This fixes #3643

Author: Roy Williams

mypy/checkexpr.py

@@ -1519,7 +1519,16 @@ def check_boolean_op(self, e: OpExpr, context: Context) -> Type:
         elif e.right_always:
             left_map = None
 
-        right_type = self.analyze_cond_branch(right_map, e.right, left_type)
+        # If right_map is None then we know mypy considers the right branch
+        # to be unreachable and therefore any errors found in the right branch
+        # should be suppressed.
+        if right_map is None:
+            self.msg.disable_errors()
+        try:
+            right_type = self.analyze_cond_branch(right_map, e.right, left_type)
+        finally:
+            if right_map is None:
+                self.msg.enable_errors()
 
         if right_map is None:
             # The boolean expression is statically known to be the left value

test-data/unit/check-isinstance.test

@@ -1187,6 +1187,8 @@ if isinstance(x, B) and isinstance(y, int):
     1() # type checking skipped
 if isinstance(y, int) and isinstance(x, B):
     1() # type checking skipped
+if isinstance(y, int) and y > 42:
+    1() # type checking skipped
 [builtins fixtures/isinstancelist.pyi]
 
 [case testReturnWithCallExprAndIsinstance]
@@ -1754,3 +1756,9 @@ if isinstance(x, str, 1): # E: Too many arguments for "isinstance"
     x = 1
     reveal_type(x) # E: Revealed type is 'builtins.int'
 [builtins fixtures/isinstancelist.pyi]
+
+[case testTypesAfterNoneFilteredOutIgnored]
+x:int = 32
+not isinstance(x, int) and x.foobar()
+
+[builtins fixtures/isinstancelist.pyi]