tuple slice should not propagate fallback (#16154)

Fixes #8776

Author: graingert

mypy/checkexpr.py

@@ -4271,7 +4271,7 @@ def visit_tuple_slice_helper(self, left_type: TupleType, slic: SliceExpr) -> Typ
 
         items: list[Type] = []
         for b, e, s in itertools.product(begin, end, stride):
-            item = left_type.slice(b, e, s)
+            item = left_type.slice(b, e, s, fallback=self.named_type("builtins.tuple"))
             if item is None:
                 self.chk.fail(message_registry.AMBIGUOUS_SLICE_OF_VARIADIC_TUPLE, slic)
                 return AnyType(TypeOfAny.from_error)

mypy/types.py

@@ -2416,7 +2416,12 @@ def copy_modified(
             items = self.items
         return TupleType(items, fallback, self.line, self.column)
 
-    def slice(self, begin: int | None, end: int | None, stride: int | None) -> TupleType | None:
+    def slice(
+        self, begin: int | None, end: int | None, stride: int | None, *, fallback: Instance | None
+    ) -> TupleType | None:
+        if fallback is None:
+            fallback = self.partial_fallback
+
         if any(isinstance(t, UnpackType) for t in self.items):
             total = len(self.items)
             unpack_index = find_unpack_in_list(self.items)
@@ -2462,7 +2467,7 @@ def slice(self, begin: int | None, end: int | None, stride: int | None) -> Tuple
                 return None
         else:
             slice_items = self.items[begin:end:stride]
-        return TupleType(slice_items, self.partial_fallback, self.line, self.column, self.implicit)
+        return TupleType(slice_items, fallback, self.line, self.column, self.implicit)
 
 
 class TypedDictType(ProperType):

test-data/unit/check-literal.test

@@ -1872,8 +1872,9 @@ reveal_type(tup2[idx3])       # N: Revealed type is "__main__.D"
 reveal_type(tup2[idx4])       # N: Revealed type is "__main__.E"
 reveal_type(tup2[idx_neg1])   # N: Revealed type is "__main__.E"
 tup2[idx5]                    # E: Tuple index out of range
-reveal_type(tup2[idx2:idx4])  # N: Revealed type is "Tuple[__main__.C, __main__.D, fallback=__main__.Tup2Class]"
-reveal_type(tup2[::idx2])     # N: Revealed type is "Tuple[__main__.A, __main__.C, __main__.E, fallback=__main__.Tup2Class]"
+reveal_type(tup2[idx2:idx4])  # N: Revealed type is "Tuple[__main__.C, __main__.D]"
+reveal_type(tup2[::idx2])     # N: Revealed type is "Tuple[__main__.A, __main__.C, __main__.E]"
+tup3: Tup2Class = tup2[:]     # E: Incompatible types in assignment (expression has type "Tuple[A, B, C, D, E]", variable has type "Tup2Class")
 [builtins fixtures/slice.pyi]
 
 [case testLiteralIntelligentIndexingTypedDict]
@@ -1977,8 +1978,8 @@ reveal_type(tup1[0::idx1])      # N: Revealed type is "Union[Tuple[__main__.A, _
 tup1[idx_bad]                   # E: Tuple index out of range
 
 reveal_type(tup2[idx1])         # N: Revealed type is "Union[__main__.B, __main__.C]"
-reveal_type(tup2[idx1:idx2])    # N: Revealed type is "Union[Tuple[__main__.B, __main__.C, fallback=__main__.Tup2Class], Tuple[__main__.B, __main__.C, __main__.D, fallback=__main__.Tup2Class], Tuple[__main__.C, fallback=__main__.Tup2Class], Tuple[__main__.C, __main__.D, fallback=__main__.Tup2Class]]"
-reveal_type(tup2[0::idx1])      # N: Revealed type is "Union[Tuple[__main__.A, __main__.B, __main__.C, __main__.D, __main__.E, fallback=__main__.Tup2Class], Tuple[__main__.A, __main__.C, __main__.E, fallback=__main__.Tup2Class]]"
+reveal_type(tup2[idx1:idx2])    # N: Revealed type is "Union[Tuple[__main__.B, __main__.C], Tuple[__main__.B, __main__.C, __main__.D], Tuple[__main__.C], Tuple[__main__.C, __main__.D]]"
+reveal_type(tup2[0::idx1])      # N: Revealed type is "Union[Tuple[__main__.A, __main__.B, __main__.C, __main__.D, __main__.E], Tuple[__main__.A, __main__.C, __main__.E]]"
 tup2[idx_bad]                   # E: Tuple index out of range
 [builtins fixtures/slice.pyi]
 [out]

test-data/unit/check-tuples.test

@@ -1681,3 +1681,13 @@ def g(t: Tuple):
     reveal_type(zip(*t))  # N: Revealed type is "typing.Iterator[builtins.tuple[Any, ...]]"
     reveal_type(zip(t))  # N: Revealed type is "typing.Iterator[Tuple[Any]]"
 [builtins fixtures/tuple.pyi]
+
+[case testTupleSubclassSlice]
+from typing import Tuple
+
+class A: ...
+
+class tuple_aa_subclass(Tuple[A, A]): ...
+
+inst_tuple_aa_subclass: tuple_aa_subclass = tuple_aa_subclass((A(), A()))[:]  # E: Incompatible types in assignment (expression has type "Tuple[A, A]", variable has type "tuple_aa_subclass")
+[builtins fixtures/tuple.pyi]