c++: Relax too strict assert in stabilize_expr [PR111160]

The case in the ticket is an ICE on invalid due to an assert in stabilize_expr,
but the underlying issue can actually trigger on this *valid* code:

=== cut here ===
struct TheClass {
  TheClass() {}
  TheClass(volatile TheClass& t) {}
  TheClass operator=(volatile TheClass& t) volatile { return t; }
};
void the_func() {
  volatile TheClass x, y, z;
  (false ? x : y) = z;
}
=== cut here ===

The problem is that stabilize_expr asserts that it returns an expression
without TREE_SIDE_EFFECTS, which can't be if the involved type is volatile.

This patch relaxes the assert to accept having TREE_THIS_VOLATILE on the
returned expression.

Successfully tested on x86_64-pc-linux-gnu.

	PR c++/111160

gcc/cp/ChangeLog:

	* tree.cc (stabilize_expr): Stabilized expressions can have
	TREE_SIDE_EFFECTS if they're volatile.

gcc/testsuite/ChangeLog:

	* g++.dg/overload/error8.C: New test.
	* g++.dg/overload/volatile2.C: New test.

3 files changed, 22 insertions, 1 deletions

diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index 28648c1..dfd4a3a 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -5969,7 +5969,7 @@ stabilize_expr (tree exp, tree* initp)
     }
   *initp = init_expr;
 
-  gcc_assert (!TREE_SIDE_EFFECTS (exp));
+  gcc_assert (!TREE_SIDE_EFFECTS (exp) || TREE_THIS_VOLATILE (exp));
   return exp;
 }
 
diff --git a/gcc/testsuite/g++.dg/overload/error8.C b/gcc/testsuite/g++.dg/overload/error8.C
new file mode 100644
index 0000000..a7e7458
--- /dev/null
+++ b/gcc/testsuite/g++.dg/overload/error8.C
@@ -0,0 +1,9 @@
+// PR c++/111160
+// { dg-do compile { target c++11 } }
+
+class TheClass {}; // { dg-error "discards|bind|discards|bind" }
+void the_func() {
+  TheClass x;
+  volatile TheClass y;
+  (false ? x : x) = y; // { dg-error "ambiguous|ambiguous" }
+}
diff --git a/gcc/testsuite/g++.dg/overload/volatile2.C b/gcc/testsuite/g++.dg/overload/volatile2.C
new file mode 100644
index 0000000..9f27357
--- /dev/null
+++ b/gcc/testsuite/g++.dg/overload/volatile2.C
@@ -0,0 +1,12 @@
+// PR c++/111160
+// { dg-do compile { target c++11 } }
+
+struct TheClass {
+  TheClass() {}
+  TheClass(volatile TheClass& t) {}
+  TheClass operator=(volatile TheClass& t) volatile { return t; }
+};
+void the_func() {
+  volatile TheClass x, y, z;
+  (false ? x : y) = z;
+}