c++: avoid ARM -Wunused-value [PR114970]

Because of the __builtin_is_constant_evaluated, maybe_constant_init in
expand_default_init fails, so the constexpr constructor isn't folded until
cp_fold, which then calls cp_build_init_expr_for_ctor, which builds a
COMPOUND_EXPR in case the enclosing expression is relying on the ARM
behavior of returning 'this'.

As in other places, avoid -Wunused-value on artificial COMPOUND_EXPR.

	PR c++/114970

gcc/cp/ChangeLog:

	* cp-gimplify.cc (cp_build_init_expr_for_ctor): Suppress warnings on
	return_this COMPOUND_EXPR.

gcc/testsuite/ChangeLog:

	* g++.dg/opt/is_constant_evaluated4.C: New test.

2 files changed, 25 insertions, 2 deletions

diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index 550cea29..f5625ab 100644
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -1199,8 +1199,11 @@ cp_build_init_expr_for_ctor (tree call, tree init)
   tree s = build_fold_indirect_ref_loc (loc, a);
   init = cp_build_init_expr (s, init);
   if (return_this)
-    init = build2_loc (loc, COMPOUND_EXPR, TREE_TYPE (call), init,
-		    fold_convert_loc (loc, TREE_TYPE (call), a));
+    {
+      init = build2_loc (loc, COMPOUND_EXPR, TREE_TYPE (call), init,
+			 fold_convert_loc (loc, TREE_TYPE (call), a));
+      suppress_warning (init);
+    }
   return init;
 }
 
diff --git a/gcc/testsuite/g++.dg/opt/is_constant_evaluated4.C b/gcc/testsuite/g++.dg/opt/is_constant_evaluated4.C
new file mode 100644
index 0000000..9650004
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/is_constant_evaluated4.C
@@ -0,0 +1,20 @@
+// PR c++/114970
+// { dg-do compile { target c++17 } }
+// { dg-additional-options "-O -Wunused-value" }
+
+struct sv
+{
+  const char* str;
+  unsigned len;
+
+  constexpr sv(const char *p): str(p), len(0)
+  {
+    if (__builtin_is_constant_evaluated ()) { len = 42; }
+  }
+};
+
+int main()
+{
+  sv s ("foo");
+  return s.len;
+}