fix[lang]: fix .vyi function body check (#4177)

previously the validation for function bodies in interfaces incorrectly
allowed other types of expressions with the `.value` member, e.g.

```vyper
@external
def some_interface_function():
    return ...
```
although the body should only be allowed to be an ellipsis expr (`...`).

this commit correctly rejects the above source code

tests/functional/syntax/test_interfaces.py

@@ -3,6 +3,7 @@
 from vyper import compiler
 from vyper.exceptions import (
     ArgumentException,
+    FunctionDeclarationException,
     InterfaceViolation,
     InvalidReference,
     InvalidType,
@@ -421,3 +422,65 @@ def test_builtins_not_found2(erc):
         compiler.compile_code(code)
     assert e.value._message == f"ethereum.ercs.{erc}"
     assert e.value._hint == f"try renaming `{erc}` to `I{erc}`"
+
+
+def test_interface_body_check(make_input_bundle):
+    interface_code = """
+@external
+def foobar():
+    return ...
+"""
+
+    input_bundle = make_input_bundle({"foo.vyi": interface_code})
+
+    code = """
+import foo as Foo
+
+implements: Foo
+
+@external
+def foobar():
+    pass
+"""
+    with pytest.raises(FunctionDeclarationException) as e:
+        compiler.compile_code(code, input_bundle=input_bundle)
+
+    assert e.value._message == "function body in an interface can only be `...`!"
+
+
+def test_interface_body_check2(make_input_bundle):
+    interface_code = """
+@external
+def foobar():
+    ...
+
+@external
+def bar():
+    ...
+
+@external
+def baz():
+    ...
+"""
+
+    input_bundle = make_input_bundle({"foo.vyi": interface_code})
+
+    code = """
+import foo
+
+implements: foo
+
+@external
+def foobar():
+    pass
+
+@external
+def bar():
+    pass
+
+@external
+def baz():
+    pass
+"""
+
+    assert compiler.compile_code(code, input_bundle=input_bundle) is not None

vyper/semantics/types/function.py

@@ -344,7 +344,11 @@ def from_vyi(cls, funcdef: vy_ast.FunctionDef) -> "ContractFunctionT":
 
         return_type = _parse_return_type(funcdef)
 
-        if len(funcdef.body) != 1 or not isinstance(funcdef.body[0].get("value"), vy_ast.Ellipsis):
+        body = funcdef.body
+
+        if len(body) != 1 or not (
+            isinstance(body[0], vy_ast.Expr) and isinstance(body[0].value, vy_ast.Ellipsis)
+        ):
             raise FunctionDeclarationException(
                 "function body in an interface can only be `...`!", funcdef
             )