On Tue, Feb 28, 2017 at 12:48 PM, Jakub Jelinek <ja...@redhat.com> wrote:
> The DR1659/DR1611 changes result in construct_virtual_base not being called,
> but unfortunately the call generated in there was the spot that caused
> mark_exp_read on the arguments passed to the vbase construction (TREE_USED
> is set on these earlier already during parsing them).  That results
> in false positive -Wunused-but-set-parameter warnings.
>
> The following patch tries to avoid the warning in that case by marking
> the arguments as read (essentially pretending they were read in the omitted
> call).  Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

I believe there's some question still about whether the DR1659/11
changes are what we want; I'll defer to Nathan on this patch.

Jason

> 2017-02-28  Jakub Jelinek  <ja...@redhat.com>
>
>         PR c++/79746
>         * init.c (emit_mem_initializers): When not constructing vbases of
>         abstract classes, mark arguments as read for
>         -Wunused-but-set-parameter.
>
>         * g++.dg/warn/Wunused-parm-9.C: New test.
>
> --- gcc/cp/init.c.jj    2017-02-28 16:24:05.000000000 +0100
> +++ gcc/cp/init.c       2017-02-28 21:08:29.158380881 +0100
> @@ -1217,6 +1217,12 @@ emit_mem_initializers (tree mem_inits)
>         /* C++14 DR1658 Means we do not have to construct vbases of
>            abstract classes.  */
>         construct_virtual_base (subobject, arguments);
> +      else
> +       /* When not constructing vbases of abstract classes, at least mark
> +          the arguments expressions as read to avoid
> +          -Wunused-but-set-parameter false positives.  */
> +       for (tree arg = arguments; arg; arg = TREE_CHAIN (arg))
> +         mark_exp_read (TREE_VALUE (arg));
>
>        if (inherited_base)
>         pop_deferring_access_checks ();
> --- gcc/testsuite/g++.dg/warn/Wunused-parm-9.C.jj       2017-02-28 
> 21:11:44.989820368 +0100
> +++ gcc/testsuite/g++.dg/warn/Wunused-parm-9.C  2017-02-28 21:10:43.000000000 
> +0100
> @@ -0,0 +1,12 @@
> +// PR c++/79746
> +// { dg-do compile }
> +// { dg-options "-Wunused-but-set-parameter" }
> +
> +struct A {
> +  A (const char *x) : a(x) {}  // { dg-bogus "set but not used" }
> +  virtual int foo () = 0;
> +  const char *a;
> +};
> +struct B : public virtual A {
> +  B (const char *x) : A(x) {}  // { dg-bogus "set but not used" }
> +};
>
>         Jakub

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