Nonpayable default functions are sometimes payable in vyper

@@ -372,3 +372,24 @@ def __default__():
     assert_tx_failed(
         lambda: w3.eth.send_transaction({"to": c.address, "value": 100, "data": "0x12345678"})
     )
+
+
+def test_batch_nonpayable(get_contract, w3, assert_tx_failed):
+    code = """
+@external
+def foo() -> bool:
+    return True
+
+@external
+def __default__():
+    pass
+    """
+
+    c = get_contract(code)
+    w3.eth.send_transaction({"to": c.address, "value": 0, "data": "0x12345678"})
+    data = bytes([1, 2, 3, 4])
+    for i in range(5):
+        calldata = "0x" + data[:i].hex()
+        assert_tx_failed(
+            lambda: w3.eth.send_transaction({"to": c.address, "value": 100, "data": calldata})
+        )

@@ -15,12 +15,13 @@ def __init__(a: uint256):
     assert c.val() == 123
 
     # Make sure the init code does not access calldata
-    opcodes = vyper.compile_code(code, ["opcodes"])["opcodes"].split(" ")
-    ir_return_idx = opcodes.index("JUMP")
+    assembly = vyper.compile_code(code, ["asm"])["asm"].split(" ")
+    ir_return_idx_start = assembly.index("{")
+    ir_return_idx_end = assembly.index("}")
 
-    assert "CALLDATALOAD" in opcodes
-    assert "CALLDATACOPY" not in opcodes[:ir_return_idx]
-    assert "CALLDATALOAD" not in opcodes[:ir_return_idx]
+    assert "CALLDATALOAD" in assembly
+    assert "CALLDATACOPY" not in assembly[:ir_return_idx_start] + assembly[ir_return_idx_end:]
+    assert "CALLDATALOAD" not in assembly[:ir_return_idx_start] + assembly[ir_return_idx_end:]
 
 
 def test_init_calls_internal(get_contract, assert_compile_failed, assert_tx_failed):

@@ -123,7 +123,24 @@ def handler_for(calldata_kwargs, default_kwargs):
 
         ret.append(["goto", sig.external_function_base_entry_label])
 
-        ret = ["if", ["eq", "_calldata_method_id", method_id], ret]
+        method_id_check = ["eq", "_calldata_method_id", method_id]
+
+        # if there is a function whose selector is 0, it won't be distinguished
+        # from the case where nil calldata is supplied, b/c calldataload loads
+        # 0s past the end of physical calldata (cf. yellow paper).
+        # since supplying 0 calldata is expected to trigger the fallback fn,
+        # we check that calldatasize > 0, which distinguishes the 0 selector
+        # from the fallback function "selector"
+        # (equiv. to "all selectors not in the selector table").
+
+        # note: cases where not enough calldata is supplied (besides
+        # calldatasize==0) are not addressed here b/c a calldatasize
+        # well-formedness check is already present in the function body
+        # as part of abi validation
+        if method_id.value == 0:
+            method_id_check = ["and", ["gt", "calldatasize", 0], method_id_check]
+
+        ret = ["if", method_id_check, ret]
         return ret
 
     ret = ["seq"]

@@ -120,9 +120,6 @@ def _runtime_ir(runtime_functions, all_sigs, global_ctx):
 
     runtime = [
         "seq",
-        # check that calldatasize is at least 4, otherwise
-        # calldataload will load zeros (cf. yellow paper).
-        ["if", ["lt", "calldatasize", 4], ["goto", "fallback"]],
         ["with", "_calldata_method_id", shr(224, ["calldataload", 0]), selector_section],
         close_selector_section,
         ["label", "fallback", ["var_list"], fallback_ir],