Fix self type check for generic NamedTuple with constrained typevar

mypy/checker.py

@@ -105,7 +105,7 @@ def __init__(self) -> None:
     IterationErrorWatcher,
     report_internal_error,
 )
-from mypy.expandtype import expand_type
+from mypy.expandtype import expand_type, expand_type_by_instance
 from mypy.literals import Key, extract_var_from_literal_hash, literal, literal_hash
 from mypy.maptype import map_instance_to_supertype
 from mypy.meet import is_overlapping_types, meet_types
@@ -1778,6 +1778,10 @@ def require_correct_self_argument(self, func: Type, defn: FuncDef) -> bool:
             return True
 
         arg_type = func.arg_types[0]
+        p_arg = get_proper_type(arg_type)
+        ref_type = get_proper_type(ref_type)
+        if isinstance(ref_type, TupleType) and isinstance(p_arg, TupleType):
+            ref_type = expand_type_by_instance(ref_type, p_arg.partial_fallback)
         if defn.is_class or defn.name == "__new__":
             ref_type = mypy.types.TypeType.make_normalized(ref_type)
         if is_same_type(arg_type, ref_type):

test-data/unit/check-namedtuple.test

@@ -1296,6 +1296,22 @@ reveal_type(foo(x, nts))  # N: Revealed type is "builtins.tuple[builtins.int | b
 [builtins fixtures/tuple.pyi]
 [typing fixtures/typing-namedtuple.pyi]
 
+[case testGenericNamedTupleWithConstrainedTypeParams]
+# https://github.com/python/mypy/issues/21453
+
+from typing import Generic, NamedTuple, TypeVar
+
+S = TypeVar("S", str, bytes)
+
+class Result(NamedTuple, Generic[S]):
+    value: S
+
+    def get_value(self) -> S:
+        reveal_type(self) # N: Revealed type is "tuple[builtins.str, fallback=__main__.Result[builtins.str]]" \
+                          # N: Revealed type is "tuple[builtins.bytes, fallback=__main__.Result[builtins.bytes]]"
+        return self.value
+[builtins fixtures/tuple.pyi]
+
 [case testGenericNamedTupleCallSyntax]
 from typing import NamedTuple, TypeVar