chore[docs]: abi function signature for default arguments (vyperlang#…

…4415)

this commit adds a note on how the function selector is calculated if
default arguments are used.

docs/control-structures.rst

@@ -48,7 +48,16 @@ External functions (marked with the ``@external`` decorator) are a part of the c
 A Vyper contract cannot call directly between two external functions. If you must do this, you can use an :ref:`interface <interfaces>`.
 
 .. note::
-    For external functions with default arguments like ``def my_function(x: uint256, b: uint256 = 1)`` the Vyper compiler will generate ``N+1`` overloaded function selectors based on ``N`` default arguments.
+    For external functions with default arguments like ``def my_function(x: uint256, b: uint256 = 1)`` the Vyper compiler will generate ``N+1`` overloaded function selectors based on ``N`` default arguments. Consequently, the ABI signature for a function (this includes interface functions) excludes optional arguments when their default values are used in the function call.
+
+    .. code-block:: vyper
+
+        from ethereum.ercs import IERC4626
+
+        @external
+        def foo(x: IERC4626):
+            extcall x.withdraw(0, self, self)   # keccak256("withdraw(uint256,address,address)")[:4] = 0xb460af94
+            extcall x.withdraw(0)               # keccak256("withdraw(uint256)")[:4] = 0x2e1a7d4d
 
 .. _structure-functions-internal:
 

docs/interfaces.rst

@@ -120,6 +120,10 @@ This imports the defined interface from the vyper file at ``an_interface.vyi`` (
 
   Prior to v0.4.0, ``implements`` required that events defined in an interface were re-defined in the "implementing" contract. As of v0.4.0, this is no longer required because events can be used just by importing them. Any events used in a contract will automatically be exported in the ABI output.
 
+.. note::
+
+  An interface function with default parameters (e.g. ``deposit(assets: uint256, receiver: address = msg.sender)``) implies that the contract being interfaced with supports these default arguments via the ABI-encoded function signatures (e.g. ``keccak256("deposit(uint256,address)")[:4]`` and ``keccak256("deposit(uint256)")[:4]``). It is the responsibility of the callee to implement the behavior associated with these defaults.
+
 Standalone Interfaces
 =====================