Insidious accidental lambda conversion

[lefticus]

Can you spot the bug?

From Ben Deane

const bool proceed = [&](){ return complexcheck; };

if (proceed) {
  // do things
}

[fcolecumberri]

That does not even compile.

gcc claims that:

<source>:3:26: error: cannot convert 'main()::<lambda()>' to 'const bool' in initialization

Besides that, is there something else are we supposed to spot?

[JohelEGP]

It probably should have been auto instead of bool, so that the conversion is attempted at the if: https://compiler-explorer.com/z/9Kxf3doqc.

bool complexcheck;
int main() {
  const auto proceed = [&](){ return complexcheck; };

  if (proceed) {
    // do things
  }
}

This does convert: https://compiler-explorer.com/z/zv9bxMrza.

int main() {
  auto _ = [](){ };
  if (_) { }
}

[lefticus]

Thanks everyone, you made me double check this.

The actual problem is the & capture. We need it to be a captureless lambda so that the conversion to function * is found, which implicit converts to bool

[lefticus]

@elbeno if you can provide more context that would be great.

[elbeno]

It was a variable template - something like this:

template <typename T>
constexpr bool thing_enabled = [] {
  if constexpr (requires { T::thing_enabled; }) {
    return T::thing_enabled;
  } else {
    return false;
  }
}; // missing immediate invocation

The intent is to immediately invoke the lambda to determine whether thing_enabled is true for a type. But we are forgetting the invocation. And the lambda doesn't capture, so it has an implicit conversion to function pointer which is always true.

For very simple cases, a conditional expression rather than an IILE could perhaps be used, but in general the logic inside the lambda doesn't have to be very complex to make this sort of thing awkward -- or impossible: if constexpr can result in different return types on different control paths -- to write as a conditional expression.

Of course adequate test coverage also makes this bug easier to find, but nevertheless it's a real-world bug formulation.

[lefticus]

Coming in episode 507