c++: Avoid -Wunused-value false positives on nullptr passed to ellipsis [PR100666]

When passing expressions with decltype(nullptr) type with side-effects to
ellipsis, we pass (void *)0 instead, but for the side-effects evaluate them
on the lhs of a COMPOUND_EXPR.  Unfortunately that means we warn about it
if the expression is a call to nodiscard marked function, even when the
result is really used, just needs to be transformed.

Fixed by adding a warning_sentinel.

2021-05-25  Jakub Jelinek  <jakub@redhat.com>

	PR c++/100666
	* call.c (convert_arg_to_ellipsis): For expressions with NULLPTR_TYPE
	and side-effects, temporarily disable -Wunused-result warning when
	building COMPOUND_EXPR.

	* g++.dg/cpp1z/nodiscard8.C: New test.
	* g++.dg/cpp1z/nodiscard9.C: New test.

3 files changed, 41 insertions, 1 deletions

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index cfccf27..3076fe6 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -8178,7 +8178,10 @@ convert_arg_to_ellipsis (tree arg, tsubst_flags_t complain)
     {
       arg = mark_rvalue_use (arg);
       if (TREE_SIDE_EFFECTS (arg))
-	arg = cp_build_compound_expr (arg, null_pointer_node, complain);
+	{
+	  warning_sentinel w(warn_unused_result);
+	  arg = cp_build_compound_expr (arg, null_pointer_node, complain);
+	}
       else
 	arg = null_pointer_node;
     }
diff --git a/gcc/testsuite/g++.dg/cpp1z/nodiscard8.C b/gcc/testsuite/g++.dg/cpp1z/nodiscard8.C
new file mode 100644
index 0000000..b5096ac
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/nodiscard8.C
@@ -0,0 +1,15 @@
+// PR c++/100666
+// { dg-do compile { target c++11 } }
+
+[[nodiscard]] decltype(nullptr) bar ();
+extern void foo (...);
+template <typename T> void qux (T);
+
+void
+baz ()
+{
+  foo (bar ());		// { dg-bogus "ignoring return value of '\[^\n\r]*', declared with attribute 'nodiscard'" }
+  bar ();		// { dg-warning "ignoring return value of '\[^\n\r]*', declared with attribute 'nodiscard'" }
+  auto x = bar ();	// { dg-bogus "ignoring return value of '\[^\n\r]*', declared with attribute 'nodiscard'" }
+  qux (bar ());		// { dg-bogus "ignoring return value of '\[^\n\r]*', declared with attribute 'nodiscard'" }
+}
diff --git a/gcc/testsuite/g++.dg/cpp1z/nodiscard9.C b/gcc/testsuite/g++.dg/cpp1z/nodiscard9.C
new file mode 100644
index 0000000..1315ccd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/nodiscard9.C
@@ -0,0 +1,22 @@
+// PR c++/100666
+// { dg-do compile { target c++11 } }
+
+struct S {};
+[[nodiscard]] S bar ();
+struct U { S s; };
+[[nodiscard]] U corge ();
+extern void foo (...);
+template <typename T> void qux (T);
+
+void
+baz ()
+{
+  foo (bar ());		// { dg-bogus "ignoring return value of '\[^\n\r]*', declared with attribute 'nodiscard'" }
+  bar ();		// { dg-warning "ignoring return value of '\[^\n\r]*', declared with attribute 'nodiscard'" }
+  auto x = bar ();	// { dg-bogus "ignoring return value of '\[^\n\r]*', declared with attribute 'nodiscard'" }
+  qux (bar ());		// { dg-bogus "ignoring return value of '\[^\n\r]*', declared with attribute 'nodiscard'" }
+  foo (corge ());	// { dg-bogus "ignoring return value of '\[^\n\r]*', declared with attribute 'nodiscard'" }
+  corge ();		// { dg-warning "ignoring return value of '\[^\n\r]*', declared with attribute 'nodiscard'" }
+  auto y = corge ();	// { dg-bogus "ignoring return value of '\[^\n\r]*', declared with attribute 'nodiscard'" }
+  qux (corge ());	// { dg-bogus "ignoring return value of '\[^\n\r]*', declared with attribute 'nodiscard'" }
+}