[cfe-dev] Undiagnosed "reference to stack memory [...] returned"

[Ted Kremenek]

I'm not certain this is a bug.  The 'const int&' causes the lifetime of the object returned from get_value() to be extended to its last use.  Does this extend to the caller of get_reference_no_warning_2() since that also returns a const reference?

On Sep 15, 2010, at 11:04 AM, Matthieu Monrocq wrote:

> Hi guys,
> 
> here is a very simple test case:
> 
>     int const& get_reference_warning()
>     {
>       int w = 0;
>       return w; // expected-warning{{reference to stack memory associated with local variable 'w' returned}}
>     }
> 
>     int get_value();
> 
>     int const& get_reference_no_warning() { return get_value(); } // Diagnosed by VC++ 2010 (haven't tested on gcc), but unnoticed by Clang
> 
>     int const& get_reference_no_warning_2()
>     {
>       int const& w2 = get_value();         // Correct, lifetime of the value is extended to the lifetime of the const reference
>       return w2;                                   // Undefined behavior: goes unnoticed by Clang, VC++ 2010 and gcc 3.4.2
>     }
> 
> I compiled it with:   clang.exe -fsyntax-only -Wall -pedantic test.cpp   (on Windows, using PowerShell)
> 
>     test.cpp(9) :  warning: reference to stack memory associated with local variable 'w' returned
>       return w; // expected-warning{{reference to stack memory associated with local variable 'w' returned}}
>          ^
>     1 warning generated.
> 
> As expected, the first return provoked a warning.
> 
> However in the two other cases, no warning is generated.
> 
> I was wondering if it would be possible for clang to diagnose those cases as well. I guess the third case is the trickier, though since the lifetime of the value is correctly extended, therre should be a way to detect that `w2` is not just an ordinary const reference. I must admit I haven't come over the Clang code base yet so it's still a bit blurry to me (by the way if someone could kindly point me to some doc for developers...)
> 
> Should I file a bug ? (or perhaps one already exists...)
> 
> Matthieu.
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