Hi!
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?
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
