Prevent in-list optimisation when the list contains starred expresssions (GH-4494)

Closes https://github.com/cython/cython/issues/3938

Cython/Compiler/Optimize.py

@@ -1347,6 +1347,10 @@ class FlattenInListTransform(Visitor.VisitorTransform, SkipDeclarations):
             # note: lhs may have side effects
             return node
 
+        if any([arg.is_starred for arg in args]):
+            # Starred arguments do not directly translate to comparisons or "in" tests.
+            return node
+
         lhs = UtilNodes.ResultRefNode(node.operand1)
 
         conds = []

tests/run/pep448_extended_unpacking.pyx

@@ -272,6 +272,24 @@ def unpack_list_keep_originals(a, b, c):
     return [*a, *b, 2, *c]
 
 
+def unpack_starred_arg_for_in_operator(x, l, m):
+    """
+    >>> l = [1,2,3]
+    >>> m = [4,5,6]
+    >>> x = 1
+    >>> unpack_starred_arg_for_in_operator(x, l, m)
+    True
+    >>> x = 10
+    >>> unpack_starred_arg_for_in_operator(x, l, m)
+    False
+    >>> unpack_starred_arg_for_in_operator(x, l, [])
+    False
+    >>> unpack_starred_arg_for_in_operator(x, [], [])
+    False
+    """
+    return x in [*l, *m]
+
+
 ###### sets