fix[lang]!: forbid calling __default__ (#4371)

This commit forbids `extcall` and `staticcall`s to `__default__`. This
was not possible before the introduction of `__interface__()` and
`__at__()`; however, now that we have the possibility of casting
a module to its interface, it is possible to actually call the
`__default__()` function, which generates a "normal" ABI-encoded
call including the method id for `__default__()`. That could cause a
selector collision with a different function at target, and also leads
to confusing behavior (`__default__` gets called when no selector
matches).

The error message recommends the user to use `raw_call()` for this
use case.

---------

Co-authored-by: cyberthirst <[email protected]>
Co-authored-by: Charles Cooper <[email protected]>

Author: 3 people

tests/functional/syntax/test_interfaces.py

@@ -3,6 +3,7 @@
 from vyper import compiler
 from vyper.exceptions import (
     ArgumentException,
+    CallViolation,
     FunctionDeclarationException,
     InterfaceViolation,
     InvalidReference,
@@ -605,7 +606,6 @@ def bar():
         compiler.compile_code(main, input_bundle=input_bundle)
 
 
-@pytest.mark.xfail
 def test_intrinsic_interfaces_default_function(make_input_bundle, get_contract):
     lib1 = """
 @external
@@ -623,6 +623,26 @@ def bar():
     """
     input_bundle = make_input_bundle({"lib1.vy": lib1})
 
-    # TODO make the exception more precise once fixed
-    with pytest.raises(Exception):  # noqa: B017
+    with pytest.raises(CallViolation):
+        compiler.compile_code(main, input_bundle=input_bundle)
+
+
+def test_intrinsic_interfaces_default_function_staticcall(make_input_bundle, get_contract):
+    lib1 = """
+@external
+@view
+def __default__() -> int128:
+    return 43
+    """
+    main = """
+import lib1
+
+@external
+def bar():
+    foo:int128 = 0
+    foo = staticcall lib1.__at__(self).__default__()
+    """
+    input_bundle = make_input_bundle({"lib1.vy": lib1})
+
+    with pytest.raises(CallViolation):
         compiler.compile_code(main, input_bundle=input_bundle)

vyper/semantics/analysis/local.py

@@ -775,6 +775,11 @@ def visit_Call(self, node: vy_ast.Call, typ: VyperType) -> None:
                     msg += f"must use the `{should}` keyword."
                     hint = f"try `{should} {node.node_source_code}`"
                     raise CallViolation(msg, hint=hint)
+
+                if func_type.is_fallback:
+                    msg = "`__default__` function cannot be called directly."
+                    msg += " If you mean to call the default function, use `raw_call`"
+                    raise CallViolation(msg)
             else:
                 if not node.is_plain_call:
                     kind = node.kind_str