fix handling of vy_ast.arg nodes

tests/functional/codegen/features/iteration/test_for_in_list.py

@@ -914,3 +914,19 @@ def foo() -> DynArray[uint256, 10]:
     """
     c = get_contract(code)
     assert c.foo() == [1, 2, 3, 4]
+
+
+def test_iterator_modification_func_arg(get_contract):
+    code = """
+@internal
+def boo(a: DynArray[uint256, 12] = [], b: DynArray[uint256, 12] = []) -> DynArray[uint256, 12]:
+    for i: uint256 in a:
+        b.append(i)
+    return b
+
+@external
+def foo() -> DynArray[uint256, 12]:
+    return self.boo([1, 2, 3])
+    """
+    c = get_contract(code)
+    assert c.foo() == [1, 2, 3]

vyper/semantics/analysis/base.py

@@ -257,7 +257,10 @@ def to_dict(self):
         # map SUBSCRIPT_ACCESS to `"$subscript_access"` (which is an identifier
         # which can't be constructed by the user)
         path = ["$subscript_access" if s is self.SUBSCRIPT_ACCESS else s for s in self.path]
-        varname = var.decl_node.target.id
+        if isinstance(var.decl_node, vy_ast.arg):
+            varname = var.decl_node.arg
+        else:
+            varname = var.decl_node.target.id
 
         decl_node = var.decl_node.get_id_dict()
         ret = {"name": varname, "decl_node": decl_node, "access_path": path}

vyper/semantics/analysis/local.py

@@ -317,7 +317,7 @@ def analyze(self):
 
         for arg in self.func.arguments:
             self.namespace[arg.name] = VarInfo(
-                arg.typ, location=location, modifiability=modifiability, decl_node=arg
+                arg.typ, location=location, modifiability=modifiability, decl_node=arg.ast_source
             )
 
         for node in self.fn_node.body: