fix[ux]: indexing empty literal arrays

tests/unit/semantics/analysis/test_array_index.py

@@ -55,6 +55,28 @@ def foo():
         analyze_module(vyper_module, dummy_input_bundle)
 
 
+def test_empty_array_index(dummy_input_bundle):
+    code = """
+a: constant(DynArray[uint256, 3]) = []
+
+@internal
+def foo():
+    b: uint256 = a[0]
+    """
+    vyper_module = parse_to_ast(code)
+    with pytest.raises(ArrayIndexException):
+        analyze_module(vyper_module, dummy_input_bundle)
+
+    code = """
+@internal
+def foo():
+    b: uint256 = empty(DynArray[uint256, 3])[0]
+    """
+    vyper_module = parse_to_ast(code)
+    with pytest.raises(ArrayIndexException):
+        analyze_module(vyper_module, dummy_input_bundle)
+
+
 @pytest.mark.parametrize("value", ["b", "self.b"])
 def test_undeclared_definition(namespace, value, dummy_input_bundle):
     code = f"""

vyper/builtins/functions.py

@@ -49,6 +49,7 @@
     StructureException,
     TypeMismatch,
     UnfoldableNode,
+    UnknownType,
     ZeroDivisionException,
 )
 from vyper.semantics.analysis.base import Modifiability, VarInfo
@@ -2229,6 +2230,18 @@ def build_IR(self, expr, args, kwargs, context):
 class Empty(TypenameFoldedFunctionT):
     _id = "empty"
 
+    def _try_fold(self, node):
+        try:
+            type_ = self.fetch_call_return(node)
+        except UnknownType:
+            raise UnfoldableNode
+
+        # special handling for dynamic arrays to catch indexing of zero arrays
+        if not isinstance(type_, DArrayT):
+            raise UnfoldableNode
+
+        return vy_ast.List.from_node(node, elements=[])
+
     def fetch_call_return(self, node):
         type_ = self.infer_arg_types(node)[0].typedef
         if isinstance(type_, HashMapT):

vyper/semantics/analysis/local.py

@@ -6,6 +6,7 @@
 from vyper import ast as vy_ast
 from vyper.ast.validation import validate_call_args
 from vyper.exceptions import (
+    ArrayIndexException,
     CallViolation,
     ExceptionList,
     FunctionDeclarationException,
@@ -920,6 +921,10 @@ def visit_Subscript(self, node: vy_ast.Subscript, typ: VyperType) -> None:
             # Arrays allow most int types as index: Take the least specific
             index_type = get_possible_types_from_node(node.slice).pop()
 
+        val = node.value.reduced()
+        if isinstance(val, vy_ast.List) and len(val.elements) == 0:
+            raise ArrayIndexException("Indexing into empty array is not allowed", node.value)
+
         self.visit(node.value, base_type)
         self.visit(node.slice, index_type)