fix[lang]: typecheck hashmap indexes with folding (vyperlang#4007)

this commit fixes a bug in typechecking of folded values for hashmap
indexes. there was previously a carveout for subscript typechecking for
arrays; however, it should not apply to hashmap indexes.

this was introduced as an edge case resulting from the semantic changes
in 56c4c9d. a related fix was applied in 75fb059, but it did not
handle hashmaps.

tests/functional/codegen/storage_variables/test_getters.py

@@ -1,3 +1,9 @@
+import pytest
+
+from vyper import compile_code
+from vyper.exceptions import OverflowException, TypeMismatch
+
+
 def test_state_accessor(get_contract):
     state_accessor = """
 y: HashMap[int128, int128]
@@ -98,3 +104,32 @@ def __init__():
         if item["type"] == "constructor":
             continue
         assert item["stateMutability"] == "view"
+
+
+@pytest.mark.parametrize(
+    "typ,index,expected_error",
+    [
+        ("uint256", "-1", TypeMismatch),
+        ("uint256", "0-1", TypeMismatch),
+        ("uint256", "0-1+1", TypeMismatch),
+        ("uint256", "2**256", OverflowException),
+        ("uint256", "2**256 // 2", OverflowException),
+        ("uint256", "2 * 2**255", OverflowException),
+        ("int256", "-2**255", TypeMismatch),
+        ("int256", "-2**256", OverflowException),
+        ("int256", "2**255", TypeMismatch),
+        ("int256", "2**256 - 5", OverflowException),
+        ("int256", "2 * 2**254", TypeMismatch),
+        ("int8", "*".join(["2"] * 7), TypeMismatch),
+    ],
+)
+def test_hashmap_index_checks(typ, index, expected_error):
+    code = f"""
+m: HashMap[{typ}, uint256]
+
+@external
+def foo():
+    self.m[{index}] = 2
+    """
+    with pytest.raises(expected_error):
+        compile_code(code)

vyper/semantics/analysis/local.py

@@ -914,14 +914,14 @@ def visit_Subscript(self, node: vy_ast.Subscript, typ: VyperType) -> None:
         else:
             base_type = get_exact_type_from_node(node.value)
 
-        # get the correct type for the index, it might
-        # not be exactly base_type.key_type
-        # note: index_type is validated in types_from_Subscript
-        index_types = get_possible_types_from_node(node.slice)
-        index_type = index_types.pop()
+        if isinstance(base_type, HashMapT):
+            index_type = base_type.key_type
+        else:
+            # Arrays allow most int types as index: Take the least specific
+            index_type = get_possible_types_from_node(node.slice).pop()
 
-        self.visit(node.slice, index_type)
         self.visit(node.value, base_type)
+        self.visit(node.slice, index_type)
 
     def visit_Tuple(self, node: vy_ast.Tuple, typ: VyperType) -> None:
         if isinstance(typ, TYPE_T):