Fix redundant cast detection for generic calls

mypy/checkexpr.py

@@ -4857,14 +4857,37 @@ def visit_cast_expr(self, expr: CastExpr) -> Type:
         )
         target_type = expr.type
         options = self.chk.options
+
+        redundant_source_type = source_type
+
+        if options.warn_redundant_casts:
+            cast_context = self.type_context[-1]
+
+            with self.msg.filter_errors() as local_errors:
+                with (
+                    self.chk.local_type_map,
+                    self.chk.binder.frame_context(can_skip=False, discard=True),
+                ):
+                    inferred_source_type = self.accept(
+                        expr.expr,
+                        type_context=cast_context,
+                        allow_none_return=True,
+                        always_allow_any=True,
+                    )
+
+            if not local_errors.has_new_errors():
+                redundant_source_type = inferred_source_type
+
         if (
             options.warn_redundant_casts
             and not is_same_type(target_type, AnyType(TypeOfAny.special_form))
-            and is_same_type(source_type, target_type)
+            and is_same_type(redundant_source_type, target_type)
         ):
             self.msg.redundant_cast(target_type, expr)
+
         if options.disallow_any_unimported and has_any_from_unimported_type(target_type):
             self.msg.unimported_type_becomes_any("Target type of cast", target_type, expr)
+
         check_for_explicit_any(
             target_type, self.chk.options, self.chk.is_typeshed_stub, self.msg, context=expr
         )

test-data/unit/check-warnings.test

@@ -12,6 +12,20 @@ c = cast(int, a)
 [out]
 main:5: error: Redundant cast to "int"
 
+[case testRedundantCastGenericReturn]
+# flags: --warn-redundant-casts
+from typing import TypeVar, cast
+
+T = TypeVar("T")
+
+def identity(xs: list[T]) -> list[T]:
+    return xs
+
+def f(xs: list[int]) -> list[int]:
+    return cast(list[int], identity(xs))
+[out]
+main:10: error: Redundant cast to "list[int]"
+
 [case testRedundantCastWithIsinstance]
 # flags: --warn-redundant-casts
 from typing import cast, Union