Re: C++ PATCH to fix missing warning (PR c++/70194)

[Jason Merrill <jason at redhat dot com>]

On 03/16/2016 08:43 PM, Martin Sebor wrote:
> > @@ -3974,6 +3974,38 @@ build_vec_cmp (tree_code code, tree type,
> >     return build3 (VEC_COND_EXPR, type, cmp, minus_one_vec, zero_vec);
> >   }
> >
> > +/* Possibly warn about an address never being NULL.  */
> > +
> > +static void
> > +warn_for_null_address (location_t location, tree op, tsubst_flags_t
> > complain)
> > +{
> ...
> > +  if (TREE_CODE (cop) == ADDR_EXPR
> > +      && decl_with_nonnull_addr_p (TREE_OPERAND (cop, 0))
> > +      && !TREE_NO_WARNING (cop))
> > +    warning_at (location, OPT_Waddress, "the address of %qD will never "
> > +        "be NULL", TREE_OPERAND (cop, 0));
> > +
> > +  if (CONVERT_EXPR_P (op)
> > +      && TREE_CODE (TREE_TYPE (TREE_OPERAND (op, 0))) == REFERENCE_TYPE)
> > +    {
> > +      tree inner_op = op;
> > +      STRIP_NOPS (inner_op);
> > +
> > +      if (DECL_P (inner_op))
> > +    warning_at (location, OPT_Waddress,
> > +            "the compiler can assume that the address of "
> > +            "%qD will never be NULL", inner_op);
>
> Since I noted the subtle differences between the phrasing of
> the various -Waddress warnings in the bug, I have to ask: what is
> the significance of the difference between the two warnings here?

The difference is that in the second case, a reference could be bound to 
a null address, but that has undefined behavior, so the compiler can 
assume it won't happen.

Jason