Fix narrowing for match case with variadic tuples (#21192)

Fixes #21189

Author: hauntsaninja

mypy/checkpattern.py

@@ -378,9 +378,15 @@ def visit_sequence_pattern(self, o: SequencePattern) -> PatternType:
             # tuples, so we instead try to narrow the entire type.
             # TODO: use more precise narrowing when possible (e.g. for identical shapes).
             new_tuple_type = TupleType(new_inner_types, current_type.partial_fallback)
-            new_type, rest_type = self.chk.conditional_types_with_intersection(
+            new_type, _ = self.chk.conditional_types_with_intersection(
                 new_tuple_type, [get_type_range(current_type)], o, default=new_tuple_type
             )
+            if (
+                star_position is not None
+                and required_patterns <= len(inner_types) - 1
+                and all(is_uninhabited(rest) for rest in rest_inner_types)
+            ):
+                rest_type = UninhabitedType()
         else:
             new_inner_type = UninhabitedType()
             for typ in new_inner_types:
@@ -460,7 +466,7 @@ def expand_starred_pattern_types(
             # so we only restore the type of the star item.
             res = []
             for i, t in enumerate(types):
-                if i != star_pos:
+                if i != star_pos or is_uninhabited(t):
                     res.append(t)
                 else:
                     res.append(UnpackType(self.chk.named_generic_type("builtins.tuple", [t])))

test-data/unit/check-python310.test

@@ -3136,6 +3136,36 @@ match m3:
         reveal_type(c3)  # N: Revealed type is "builtins.int"
 [builtins fixtures/tuple.pyi]
 
+[case testMatchSequencePatternVariadicTuple]
+# flags: --strict-equality --warn-unreachable
+from typing_extensions import Unpack
+
+def f1(m: tuple[int, Unpack[tuple[str, ...]], int]) -> None:
+    match m:
+        case (a1, b1):
+            reveal_type(m)  # N: Revealed type is "tuple[builtins.int, builtins.int]"
+        case (a2, b2, c2):
+            reveal_type(m)  # N: Revealed type is "tuple[builtins.int, builtins.str, builtins.int]"
+        case (a3, b3, c3, d3):
+            reveal_type(m)  # N: Revealed type is "tuple[builtins.int, builtins.str, builtins.str, builtins.int]"
+        case (a4, *b4, c4):
+            reveal_type(m)  # N: Revealed type is "tuple[builtins.int, Unpack[builtins.tuple[builtins.str, ...]], builtins.int]"
+        case _:
+            reveal_type(m)  # E: Statement is unreachable
+
+
+def f2(m: tuple[int] | tuple[str, str] | tuple[int, Unpack[tuple[str, ...]], int]):
+    match m:
+        case (x,):
+            reveal_type(m)  # N: Revealed type is "tuple[builtins.int]"
+        case (x, y):
+            reveal_type(m)  # N: Revealed type is "tuple[builtins.str, builtins.str] | tuple[builtins.int, builtins.int]"
+        case (x, y, z):
+            reveal_type(m)  # N: Revealed type is "tuple[builtins.int, builtins.str, builtins.int]"
+        case _:
+            reveal_type(m)  # N: Revealed type is "tuple[builtins.int, Unpack[builtins.tuple[builtins.str, ...]], builtins.int]"
+[builtins fixtures/tuple.pyi]
+
 [case testMatchSequencePatternTypeVarTupleNotTooShort]
 # flags: --strict-equality --warn-unreachable
 from typing import Tuple