Avoid false unreachable and redundant-expr warnings in loops.

Fixes python#18348
Fixes python#13973
Fixes python#11612
Fixes python#8721
Fixes python#8865
Fixes python#7204

I manually checked all the listed issues.  Some of them were already partly fixed by python#18180.

Author: tyralla

mypy/checker.py

@@ -584,24 +584,44 @@ def accept_loop(
         *,
         exit_condition: Expression | None = None,
     ) -> None:
-        """Repeatedly type check a loop body until the frame doesn't change.
-        If exit_condition is set, assume it must be False on exit from the loop.
+        """Repeatedly type check a loop body until the frame doesn't change."""
 
-        Then check the else_body.
-        """
-        # The outer frame accumulates the results of all iterations
+        # The outer frame accumulates the results of all iterations:
         with self.binder.frame_context(can_skip=False, conditional_frame=True):
+
+            # Check for potential decreases in the number of partial types so as not to stop the
+            # iteration too early:
             partials_old = sum(len(pts.map) for pts in self.partial_types)
+
+            # Disable error types that we cannot safely identify in intermediate iteration steps:
+            warn_unreachable = self.options.warn_unreachable
+            if warn_unreachable:
+                self.options.warn_unreachable = False
+            warn_redundant = codes.REDUNDANT_EXPR in self.options.enabled_error_codes
+            if warn_redundant:
+                self.options.enabled_error_codes.remove(codes.REDUNDANT_EXPR)
+
             while True:
                 with self.binder.frame_context(can_skip=True, break_frame=2, continue_frame=1):
                     self.accept(body)
                 partials_new = sum(len(pts.map) for pts in self.partial_types)
                 if (partials_new == partials_old) and not self.binder.last_pop_changed:
                     break
                 partials_old = partials_new
+
+            # If necessary, reset the modified options and make up for the postponed error checks:
+            if warn_unreachable or warn_redundant:
+                self.options.warn_unreachable = warn_unreachable
+                self.options.enabled_error_codes.add(codes.REDUNDANT_EXPR)
+                with self.binder.frame_context(can_skip=True, break_frame=2, continue_frame=1):
+                    self.accept(body)
+
+            # If exit_condition is set, assume it must be False on exit from the loop:
             if exit_condition:
                 _, else_map = self.find_isinstance_check(exit_condition)
                 self.push_type_map(else_map)
+
+            # Check the else body:
             if else_body:
                 self.accept(else_body)
 

test-data/unit/check-narrowing.test

@@ -2390,3 +2390,29 @@ class A:
                 z.append(1)
 
 [builtins fixtures/primitives.pyi]
+
+[case testAvoidFalseUnreachableInLoop]
+# flags: --warn-unreachable --python-version 3.11
+
+def f() -> int | None: ...
+def b() -> bool: ...
+
+x: int | None
+x = 1
+while x is not None or b():
+    x = f()
+
+[builtins fixtures/bool.pyi]
+
+[case testAvoidFalseRedundantExprInLoop]
+# flags: --enable-error-code redundant-expr --python-version 3.11
+
+def f() -> int | None: ...
+def b() -> bool: ...
+
+x: int | None
+x = 1
+while x is not None and b():
+    x = f()
+
+[builtins fixtures/primitives.pyi]