[PATCH] handle "invisible" reference in -Wdangling-pointer (PR104436)

[Jason Merrill]

On 2/8/22 16:59, Martin Sebor wrote:
> Transforming a by-value arguments to by-reference as GCC does for some
> class types can trigger -Wdangling-pointer when the argument is used
> to store the address of a local variable.  Since the stored value is
> not accessible in the caller the warning is a false positive.
> 
> The attached patch handles this case by excluding PARM_DECLs with
> the DECL_BY_REFERENCE bit set from consideration.
> 
> While testing the patch I noticed some instances of the warning are
> uninitentionally duplicated as the pass runs more than once.  To avoid
> that, I also introduce warning suppression into the handler for this
> instance of the warning.  (There might still be others.)

The second test should verify that we do warn about returning 't' from a 
function; we don't want to ignore the DECL_BY_REFERENCE RESULT_DECL.

> +	  tree var = SSA_NAME_VAR (lhs_ref.ref);
> +	  if (DECL_BY_REFERENCE (var))
> +	    /* Avoid by-value arguments transformed into by-reference.  */
> +	    continue;

I wonder if we can we express this property of invisiref parms somewhere 
more general?  I imagine optimizations would find it useful as well. 
Could pointer_query somehow treat the reference as pointing to a 
function-local object?

I previously tried to express this by marking the reference as 
'restrict', but that was wrong 
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97474).

Jason