[gcc r10-10614] c++: Avoid -Wunused-value false positives on nullptr passed to ellipsis [PR100666]

[Jakub Jelinek <jakub@gcc.gnu.org>]

https://gcc.gnu.org/g:589a0f34b1d176b62b064c9815c341b6abf48a2b

commit r10-10614-g589a0f34b1d176b62b064c9815c341b6abf48a2b
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue May 25 17:24:38 2021 +0200

    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:
---
 gcc/cp/call.c                           |  5 ++++-
 gcc/testsuite/g++.dg/cpp1z/nodiscard8.C | 15 +++++++++++++++
 gcc/testsuite/g++.dg/cpp1z/nodiscard9.C | 22 ++++++++++++++++++++++
 3 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index f636ef4cb03..29e4a52b6c4 100644
--- 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
index 00000000000..b5096acbea1
--- /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 00000000000..1315ccdbf7c
--- /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'" }
+}