Suppress any errors found in a boolean expression after filtering out all possible types. (#3666)

This fixes #3643

Author: rowillia

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]

test-data/unit/check-optional.test

@@ -548,6 +548,21 @@ if x is not None:
 
 [builtins fixtures/ops.pyi]
 
+[case testOptionalTypeNarrowedInBooleanStatement]
+from typing import Optional
+
+x: Optional[int] = None
+x is not None and x + 42
+x is not None and x + '42'  # E: Unsupported operand types for + ("int" and "str")
+[builtins fixtures/isinstance.pyi]
+
+[case testInvalidBooleanBranchIgnored]
+from typing import Optional
+
+x = None
+x is not None and x + 42
+[builtins fixtures/isinstance.pyi]
+
 [case testOptionalLambdaInference]
 from typing import Optional, Callable
 f = None # type: Optional[Callable[[int], None]]