Fix handling of tuple type context with unpacks (#16444)

Fixes #16425

Fix is straightforward.

Author: ilevkivskyi

mypy/checkexpr.py

@@ -4902,7 +4902,7 @@ def tuple_context_matches(self, expr: TupleExpr, ctx: TupleType) -> bool:
             return len([e for e in expr.items if not isinstance(e, StarExpr)]) <= len(ctx.items)
         # For variadic context, the only easy case is when structure matches exactly.
         # TODO: try using tuple type context in more cases.
-        if len([e for e in expr.items if not isinstance(e, StarExpr)]) != 1:
+        if len([e for e in expr.items if isinstance(e, StarExpr)]) != 1:
             return False
         expr_star_index = next(i for i, lv in enumerate(expr.items) if isinstance(lv, StarExpr))
         return len(expr.items) == len(ctx.items) and ctx_unpack_index == expr_star_index
@@ -4941,6 +4941,9 @@ def visit_tuple_expr(self, e: TupleExpr) -> Type:
         if type_context_items is not None:
             unpack_in_context = find_unpack_in_list(type_context_items) is not None
         seen_unpack_in_items = False
+        allow_precise_tuples = (
+            unpack_in_context or PRECISE_TUPLE_TYPES in self.chk.options.enable_incomplete_feature
+        )
 
         # Infer item types.  Give up if there's a star expression
         # that's not a Tuple.
@@ -4981,10 +4984,7 @@ def visit_tuple_expr(self, e: TupleExpr) -> Type:
                         # result in an error later, just do something predictable here.
                         j += len(tt.items)
                 else:
-                    if (
-                        PRECISE_TUPLE_TYPES in self.chk.options.enable_incomplete_feature
-                        and not seen_unpack_in_items
-                    ):
+                    if allow_precise_tuples and not seen_unpack_in_items:
                         # Handle (x, *y, z), where y is e.g. tuple[Y, ...].
                         if isinstance(tt, Instance) and self.chk.type_is_iterable(tt):
                             item_type = self.chk.iterable_item_type(tt, e)

test-data/unit/check-typevar-tuple.test

@@ -2225,3 +2225,13 @@ bar(*keys, 1)  # OK
 reveal_type(baz(keys, 1))  # N: Revealed type is "builtins.object"
 reveal_type(baz(*keys, 1))  # N: Revealed type is "builtins.int"
 [builtins fixtures/tuple.pyi]
+
+[case testVariadicTupleContextNoCrash]
+from typing import Tuple, Unpack
+
+x: Tuple[int, Unpack[Tuple[int, ...]]] = ()  # E: Incompatible types in assignment (expression has type "Tuple[()]", variable has type "Tuple[int, Unpack[Tuple[int, ...]]]")
+y: Tuple[int, Unpack[Tuple[int, ...]]] = (1, 2)
+z: Tuple[int, Unpack[Tuple[int, ...]]] = (1,)
+w: Tuple[int, Unpack[Tuple[int, ...]]] = (1, *[2, 3, 4])
+t: Tuple[int, Unpack[Tuple[int, ...]]] = (1, *(2, 3, 4))
+[builtins fixtures/tuple.pyi]