[Bug c++/102223] no warning when calling member function on dangling reference

[federico.kircheis at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102223

--- Comment #6 from Federico Kircheis <federico.kircheis at gmail dot com> ---
> are you expecting this to go under an existing warning flag, or a new one?

Ideally -Wall, but there might already be some flags related to dangling
pointers and references.

> Your compiler explorer link doesn't enable optimizations.

That's true, but if you look at the assembly there is no function call to
std::move, thus I assumed that contrary to another (non-inline) function call,
a diagnostic would have been possible even without optimizations.
But there is no warning even with optimizations enabled.


> I assume you already know this, but the original testcase in comment 0 is perfectly valid. The static_cast creates a new temporary, which *does* get its lifetime extended.


Yes, should have made it clearer in comment #1, sorry.
Unfortunately I realized after I wrote the message that I copied the wrong
snippet (and typed thus a wrong explanation), and I could not edited it.

> With inlining, in theory it would be possible to diagnose the invalid access, but other compilers don't do it either. It would be nice, but I'm not sure it's likely to happen any time soon.

I noticed too that other compiler do not do this diagnostic.
Too bad.


> GCC doesn't even get it right in constexpr contexts, see PR 70331 and the linked bugs in the See Also field.

Ouch.


> PR 96780 comment 2 could potentially help for the specific case of std::move, but not the general case.


AS I had a discussion explicitly about std::move/xvalues and lifetime
extension, I'll say it's a good start.
And it confused me until I realized std::move is a function, and thus obviously
destroys lifetime extension in the example I gave, while the cast not.


> N.B AddressSanitizer will diagnose this at runtime.
+1