check_boolean_op: clean up semanal/type-based reachability (#10566)

mypy/checkexpr.py

@@ -2776,38 +2776,34 @@ def check_boolean_op(self, e: OpExpr, context: Context) -> Type:
 
         assert e.op in ('and', 'or')  # Checked by visit_op_expr
 
-        if e.op == 'and':
+        left_map: mypy.checker.TypeMap
+        right_map: mypy.checker.TypeMap
+        if e.right_always:
+            left_map, right_map = None, {}
+        elif e.right_unreachable:
+            left_map, right_map = {}, None
+        elif e.op == 'and':
             right_map, left_map = self.chk.find_isinstance_check(e.left)
-            restricted_left_type = false_only(left_type)
-            result_is_left = not left_type.can_be_true
         elif e.op == 'or':
             left_map, right_map = self.chk.find_isinstance_check(e.left)
-            restricted_left_type = true_only(left_type)
-            result_is_left = not left_type.can_be_false
 
         # If left_map is None then we know mypy considers the left expression
         # to be redundant.
-        #
-        # Note that we perform these checks *before* we take into account
-        # the analysis from the semanal phase below. We assume that nodes
-        # marked as unreachable during semantic analysis were done so intentionally.
-        # So, we shouldn't report an error.
-        if codes.REDUNDANT_EXPR in self.chk.options.enabled_error_codes:
-            if left_map is None:
-                self.msg.redundant_left_operand(e.op, e.left)
-
-        # Note that we perform these checks *before* we take into account
-        # the analysis from the semanal phase below. We assume that nodes
-        # marked as unreachable during semantic analysis were done so intentionally.
-        # So, we shouldn't report an error.
-        if self.chk.should_report_unreachable_issues():
-            if right_map is None:
-                self.msg.unreachable_right_operand(e.op, e.right)
-
-        if e.right_unreachable:
-            right_map = None
-        elif e.right_always:
-            left_map = None
+        if (
+            codes.REDUNDANT_EXPR in self.chk.options.enabled_error_codes
+            and left_map is None
+            # don't report an error if it's intentional
+            and not e.right_always
+        ):
+            self.msg.redundant_left_operand(e.op, e.left)
+
+        if (
+            self.chk.should_report_unreachable_issues()
+            and right_map is None
+            # don't report an error if it's intentional
+            and not e.right_unreachable
+        ):
+            self.msg.unreachable_right_operand(e.op, e.right)
 
         # 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
@@ -2824,6 +2820,13 @@ def check_boolean_op(self, e: OpExpr, context: Context) -> Type:
             assert right_map is not None  # find_isinstance_check guarantees this
             return right_type
 
+        if e.op == 'and':
+            restricted_left_type = false_only(left_type)
+            result_is_left = not left_type.can_be_true
+        elif e.op == 'or':
+            restricted_left_type = true_only(left_type)
+            result_is_left = not left_type.can_be_false
+
         if isinstance(restricted_left_type, UninhabitedType):
             # The left operand can never be the result
             return right_type

mypy/nodes.py

@@ -1737,9 +1737,9 @@ class OpExpr(Expression):
     right = None  # type: Expression
     # Inferred type for the operator method type (when relevant).
     method_type = None  # type: Optional[mypy.types.Type]
-    # Is the right side going to be evaluated every time?
+    # Per static analysis only: Is the right side going to be evaluated every time?
     right_always = False
-    # Is the right side unreachable?
+    # Per static analysis only: Is the right side unreachable?
     right_unreachable = False
 
     def __init__(self, op: str, left: Expression, right: Expression) -> None:

test-data/unit/check-unreachable-code.test

@@ -615,6 +615,27 @@ if MYPY or mypy_only:
     pass
 [builtins fixtures/ops.pyi]
 
+[case testSemanticAnalysisFalseButTypeNarrowingTrue]
+# flags: --always-false COMPILE_TIME_FALSE
+from typing import Literal
+
+indeterminate: bool
+COMPILE_TIME_FALSE: Literal[True]  # type-narrowing: mapped in 'if' only
+a = COMPILE_TIME_FALSE or indeterminate
+reveal_type(a)  # N: Revealed type is "builtins.bool"
+b = indeterminate or COMPILE_TIME_FALSE
+reveal_type(b)  # N: Revealed type is "Union[builtins.bool, Literal[True]]"
+[typing fixtures/typing-medium.pyi]
+
+[case testSemanticAnalysisTrueButTypeNarrowingFalse]
+# flags: --always-true COMPILE_TIME_TRUE
+indeterminate: bool
+COMPILE_TIME_TRUE: None  # type narrowed to `else` only
+a = COMPILE_TIME_TRUE or indeterminate
+reveal_type(a)  # N: Revealed type is "None"
+b = indeterminate or COMPILE_TIME_TRUE
+reveal_type(b)  # N: Revealed type is "Union[builtins.bool, None]"
+
 [case testConditionalAssertWithoutElse]
 import typing