From 12da260f7cabb55b81ea94f2399be40a8ebe58cc Mon Sep 17 00:00:00 2001
From: Charles Cooper <cooper.charles.m@gmail.com>
Date: Mon, 21 Oct 2024 19:51:06 -0400
Subject: [PATCH] feat[lang]: reject `implements` for empty interfaces

`implements: <interface>` for an empty interface likely indicates a bug
in user code, reject it.
---
 .../syntax/modules/test_implements.py         | 21 +++++++++++++++++++
 vyper/semantics/types/module.py               |  4 ++++
 2 files changed, 25 insertions(+)

diff --git a/tests/functional/syntax/modules/test_implements.py b/tests/functional/syntax/modules/test_implements.py
index c292e198d9..4144b0f28b 100644
--- a/tests/functional/syntax/modules/test_implements.py
+++ b/tests/functional/syntax/modules/test_implements.py
@@ -1,4 +1,7 @@
+import pytest
+
 from vyper.compiler import compile_code
+from vyper.exceptions import StructureException
 
 
 def test_implements_from_vyi(make_input_bundle):
@@ -49,3 +52,21 @@ def foo():  # implementation
     input_bundle = make_input_bundle({"some_interface.vyi": vyi, "lib1.vy": lib1, "lib2.vy": lib2})
 
     assert compile_code(main, input_bundle=input_bundle) is not None
+
+
+def test_implements_empty_vyi(make_input_bundle, tmp_path):
+    vyi = ""
+    input_bundle = make_input_bundle({"some_interface.vyi": vyi})
+    main = """
+import some_interface
+
+implements: some_interface
+    """
+    with pytest.raises(StructureException) as e:
+        _ = compile_code(main, input_bundle=input_bundle)
+
+    vyi_path = (tmp_path / "some_interface.vyi").as_posix()
+    assert (
+        e.value._message
+        == f"Tried to implement `{vyi_path}`, but it has no functions to implement!"
+    )
diff --git a/vyper/semantics/types/module.py b/vyper/semantics/types/module.py
index dabeaf21b6..7ba73da87f 100644
--- a/vyper/semantics/types/module.py
+++ b/vyper/semantics/types/module.py
@@ -112,6 +112,10 @@ def _ctor_modifiability_for_call(self, node: vy_ast.Call, modifiability: Modifia
     def validate_implements(
         self, node: vy_ast.ImplementsDecl, functions: dict[ContractFunctionT, vy_ast.VyperNode]
     ) -> None:
+        if len(self.functions) == 0:
+            msg = f"Tried to implement `{self}`, but it has no functions to implement!"
+            raise StructureException(msg, node)
+
         # only external functions can implement interfaces
         fns_by_name = {fn_t.name: fn_t for fn_t in functions.keys()}