Integer bounds error in Vyper

Impact

in the following code, the return of <iface>.returns_int128() is not validated to fall within the bounds of int128. as of v0.3.0, <iface>.returns_int128() is validated in simple expressions, but not complex expressions.

interface iface:
    def returns_int128() -> int128: view
    def returns_Bytes33() -> Bytes[33]: view

x: iface
 
@external
def call_out():
    x: int128 = self.x.returns_int128()  # affected, <0.3.0
    y: uint256 = convert(self.x.returns_int128(), uint256)  # affected, <0.3.2
    z: Bytes[33] = concat(self.x.returns_Bytes33(), b"")  # affected >= 0.3.0, <0.3.2

Patches

0.3.2 (as of https://github.com/vyperlang/vyper/commit/049dbdc647b2ce838fae7c188e6bb09cf16e470b)

Workarounds

Break up operations involving external calls into multiple statements. For instance, instead of the example above, use

x: int128 = self.x.returns_int128()
y: uint256 = convert(x, uint256)