consider redundant-casts, too

test case from @auntsaninja (slightly simplified)

Author: tyralla

mypy/checker.py

@@ -600,7 +600,8 @@ def accept_loop(
             # on without bound otherwise)
             widened_old = len(self.widened_vars)
 
-            # one set of `unreachable` and `redundant-expr`errors per iteration step:
+            # one set of `unreachable`, `redundant-expr`, and `redundant-casts` errors
+            # per iteration step:
             uselessness_errors = []
             # one set of unreachable line numbers per iteration step:
             unreachable_lines = []
@@ -640,8 +641,8 @@ def accept_loop(
                 if iter == 20:
                     raise RuntimeError("Too many iterations when checking a loop")
 
-            # Report only those `unreachable` and `redundant-expr` errors that could not
-            # be ruled out in any iteration step:
+            # Report only those `unreachable`, `redundant-expr`, and `redundant-casts`
+            # errors that could not be ruled out in any iteration step:
             persistent_uselessness_errors = set()
             for candidate in set(itertools.chain(*uselessness_errors)):
                 if all(

mypy/errors.py

@@ -222,8 +222,8 @@ def filtered_errors(self) -> list[ErrorInfo]:
 
 class LoopErrorWatcher(ErrorWatcher):
     """Error watcher that filters and separately collects `unreachable` errors,
-    `redundant-expr` errors, and revealed types when analysing loops iteratively
-    to help avoid making too-hasty reports."""
+    `redundant-expr` and `redundant-casts` errors, and revealed types when analysing
+    loops iteratively to help avoid making too-hasty reports."""
 
     # Meaning of the tuple items: ErrorCode, message, line, column, end_line, end_column:
     uselessness_errors: set[tuple[ErrorCode, str, int, int, int, int]]
@@ -254,7 +254,7 @@ def __init__(
 
     def on_error(self, file: str, info: ErrorInfo) -> bool:
 
-        if info.code in (codes.UNREACHABLE, codes.REDUNDANT_EXPR):
+        if info.code in (codes.UNREACHABLE, codes.REDUNDANT_EXPR, codes.REDUNDANT_CAST):
             self.uselessness_errors.add(
                 (info.code, info.message, info.line, info.column, info.end_line, info.end_column)
             )

test-data/unit/check-narrowing.test

@@ -2382,6 +2382,28 @@ while True:
 
 [builtins fixtures/bool.pyi]
 
+[case testAvoidFalseRedundantCastInLoops]
+# flags: --warn-redundant-casts
+
+from typing import Callable, cast, Union
+
+ProcessorReturnValue = Union[str, int]
+Processor = Callable[[str], ProcessorReturnValue]
+
+def main_cast(p: Processor) -> None:
+    ed: ProcessorReturnValue
+    ed = cast(str, ...)
+    while True:
+        ed = p(cast(str, ed))
+
+def main_no_cast(p: Processor) -> None:
+    ed: ProcessorReturnValue
+    ed = cast(str, ...)
+    while True:
+        ed = p(ed)  # E: Argument 1 has incompatible type "Union[str, int]"; expected "str"
+
+[builtins fixtures/bool.pyi]
+
 [case testAvoidFalseUnreachableInLoop1]
 # flags: --warn-unreachable --python-version 3.11