Impact

when a calling an external contract with no return value, the contract address could be evaluated twice. this is usually only an efficiency problem, but if evaluation of the contract address has side effects, it could result in double evaluation of the side effects.

in the following example, Foo(msg.sender).bar() is the contract address for the following call (to .foo()), and could get evaluated twice

interface Foo:
    def foo(): nonpayable
    def bar() -> address: nonpayable

@external
def do_stuff():
    Foo(Foo(msg.sender).bar()).foo()

Patches

6b4d8ff

Workarounds

assign contract addresses to variables. the above example would change to

@external
def do_stuff():
    t: Foo = Foo(msg.sender).bar()
    t.foo()

References

For more information