Should probably make it explicit in this case, something like: if (const auto& [is_successful, error_message] = foo(n); is_successful)

In a more normal scenario you'd expect to use std::expected here rather than a custom struct with an operator bool.

The return value of foo(n), converted to bool, acts as the condition variable…

Yeah I wouldn't like this in a code review. Add one more line of code so I don't have to investigate foo().

That's my fundamental gripe with C++

int i = 0;

function_0(i,...);

...

function_9(i,...);

which one changes i? It's not obvious in a code review due to default mutable references.