c++: copy elision w/ obj arg and static memfn call [PR110441]

Here the call A().f() is represented as a COMPOUND_EXPR whose first
operand is the otherwise unused object argument A() and second operand
is the call result (both are TARGET_EXPRs).  Within the return statement,
this outermost COMPOUND_EXPR ends up foiling the copy elision check in
build_special_member_call, resulting in us introducing a bogus call to the
deleted move constructor.  (Within the variable initialization, which goes
through ocp_convert instead of convert_for_initialization, we've already
been eliding the copy -- despite the outermost COMPOUND_EXPR -- ever since
r10-7410-g72809d6fe8e085 made ocp_convert look through COMPOUND_EXPR).

In contrast I noticed '(A(), A::f())' (which should be equivalent to
the above call) is represented with the COMPOUND_EXPR inside the RHS's
TARGET_EXPR initializer thanks to a special case in cp_build_compound_expr.

So this patch fixes this by making keep_unused_object_arg use
cp_build_compound_expr as well.

	PR c++/110441

gcc/cp/ChangeLog:

	* call.cc (keep_unused_object_arg): Use cp_build_compound_expr
	instead of building a COMPOUND_EXPR directly.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/elide8.C: New test.

2 files changed, 26 insertions, 1 deletions

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 15a3d6f..976330c 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -5216,7 +5216,7 @@ keep_unused_object_arg (tree result, tree obj, tree fn)
   if (TREE_THIS_VOLATILE (a))
     a = build_this (a);
   if (TREE_SIDE_EFFECTS (a))
-    return build2 (COMPOUND_EXPR, TREE_TYPE (result), a, result);
+    return cp_build_compound_expr (a, result, tf_error);
   return result;
 }
 
diff --git a/gcc/testsuite/g++.dg/cpp1z/elide8.C b/gcc/testsuite/g++.dg/cpp1z/elide8.C
new file mode 100644
index 0000000..7d471be
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/elide8.C
@@ -0,0 +1,25 @@
+// PR c++/110441
+// { dg-do compile { target c++11 } }
+
+struct immovable {
+  immovable(immovable &&) = delete;
+};
+
+struct A {
+  static immovable f();
+};
+
+immovable f() {
+  immovable m = A().f(); // { dg-error "deleted" "" { target c++14_down } }
+  return A().f(); // { dg-error "deleted" "" { target c++14_down } }
+}
+
+struct B {
+  A* operator->();
+};
+
+immovable g() {
+  B b;
+  immovable m = b->f(); // { dg-error "deleted" "" { target c++14_down } }
+  return b->f(); // { dg-error "deleted" "" { target c++14_down } }
+}