How to annotate an owner properly?

[brevzin]

Here's a simple example:

#include <vector>
#include <string>

void use(char);
auto f() -> std::vector<std::string>;

int main() {
    for (char c : f()[1]) {
        use(c);
    }
}

This, correctly and awesomely, issues a warning on the for loop because of the object being destroyed. Cool.

Now let's say I have my own custom vector of strings, which I'm calling VS because I'm not creative:

#include <vector>
#include <string>

void use(char);

struct [[gsl::Owner]] VS {
    auto operator[](size_t i) -> std::string&;
    auto begin() -> std::vector<std::string>::iterator;
    auto end() -> std::vector<std::string>::iterator;

    std::vector<std::string> v;
};

auto f() -> VS;

int main() {
    for (char c : f()[1]) {
        use(c);
    }
}

This does not warn, even though it's basically the same example and I tried to annotate VS as being an owner. Is there some way to still get a warning for such code?

Obligatory compiler explorer link.

[mgehre]

This seems to be an issue with determining the pointee type of the Owner VS.
Adding some debugging and an extra member makes it correctly deduce the pointee type, see https://godbolt.org/z/Erb77o

I thought that [[gsl::Owner(std::string)]] was a way to manually specify that, but it didn't work.
Also declaring operator[] is supposed to be enough to correctly determine the pointee type.

I'll leave this bug open until that is fixed.

[brevzin]

Also declaring operator[] is supposed to be enough to correctly determine the pointee type.

I think if the type is a range (has begin/end), you should be able to use that too?

[mgehre]

Yes, it should. The respective code is in https://github.com/mgehre/llvm-project/blob/lifetime/clang/lib/Analysis/LifetimeTypeCategory.cpp#L332

[mgehre]

For some reason, it doesn't recognize std::vector<std::string>::iterator as Pointer.

[mgehre]

It seems to be an issue when std::vector<std::string>::begin is not used and thus not instantiated, see
https://godbolt.org/z/6bbqKT