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.

(cherry picked from commit ad52d89808a947264397e920d7483090d4108f7b)

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index f636ef4cb03ff0d99fc9c7d707ea760b03c2072f..29e4a52b6c458397daa2dba3c7097bd5b1cc5ade 100644 (file)
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -7968,7 +7968,10 @@ 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 (file)
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 (file)
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'" }
+}