[libcxx-commits] [libcxx] Add [[clang::lifetimebound]] to numerous functions in libc++ include headers (PR #112751)

[Haojian Wu via libcxx-commits]

hokein wrote:

> I seem to be encountering a strange situation where this code gives a dangling reference error when I annotate `std::vector::operator[]` with `[[clang::lifetimebound]]`:
> 
> ```c++
> #include <string_view>
> #include <vector>
> 
> std::string_view foo() {
>     std::vector<std::string_view> v(1);
>     return v[0];
> }
> ```
> 
> This doesn't trigger if I attempt to mimic `std::vector` with my own type, which makes me think there's a bug somewhere in Clang's built-in analysis that special-cases `std::vector`...
> 
> Anybody know what is going on?

This looks like a bug in Clang's lifetime analysis when `lifetimebound` interacts with `gsl::Pointer`.

```cpp
class [[gsl::Pointer]] FooPointer {};

template<typename T>
class [[gsl::Owner]] Container {
public:
  const T& get() const [[clang::lifetimebound]];
};

FooPointer abc2() {
  Container<FooPointer> foo;
  return foo.get(); // Clang emits a return-stack-address warning on `foo`.
}
```

https://godbolt.org/z/ffYjPa7vs, 
If you remove any of `gsl::Pointer`, `gsl::Owner`, or `clang::lifetimebound`, the diagnostic disappears.
 

https://github.com/llvm/llvm-project/pull/112751