[gcc r15-3921] c++/coro: prevent ICV_STATEMENT diagnostics in temp promotion [PR116502]

[Arsen Arsenovic]

https://gcc.gnu.org/g:05e4f07cad1eacf869c10622cae2a9cdee3b6a7a

commit r15-3921-g05e4f07cad1eacf869c10622cae2a9cdee3b6a7a
Author: Arsen Arsenović <arsen@aarsen.me>
Date:   Wed Aug 28 21:59:18 2024 +0200

    c++/coro: prevent ICV_STATEMENT diagnostics in temp promotion [PR116502]
    
    If such a diagnostic is necessary, it has already been emitted,
    otherwise, it is not correct and emitting it here is inactionable by the
    user, and bogus.
    
            PR c++/116502
    
    gcc/cp/ChangeLog:
    
            * coroutines.cc (maybe_promote_temps): Convert temporary
            initializers to void without complaining.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/coroutines/maybe-unused-1.C: New test.
            * g++.dg/coroutines/pr116502.C: New test.

Diff:
---
 gcc/cp/coroutines.cc                             | 12 ++++++---
 gcc/testsuite/g++.dg/coroutines/maybe-unused-1.C | 33 ++++++++++++++++++++++++
 gcc/testsuite/g++.dg/coroutines/pr116502.C       | 33 ++++++++++++++++++++++++
 3 files changed, 75 insertions(+), 3 deletions(-)

diff --git a/gcc/cp/coroutines.cc b/gcc/cp/coroutines.cc
index 4c7ea1dd3216..50904e0d004e 100644
--- a/gcc/cp/coroutines.cc
+++ b/gcc/cp/coroutines.cc
@@ -3203,7 +3203,13 @@ maybe_promote_temps (tree *stmt, void *d)
 	 to run the initializer.
 	 If the initializer is a conditional expression, we need to collect
 	 and declare any promoted variables nested within it.  DTORs for such
-	 variables must be run conditionally too.  */
+	 variables must be run conditionally too.
+
+	 Since here we're synthetically processing code here, we've already
+	 emitted any Wunused-result warnings.  Below, however, we call
+	 finish_expr_stmt, which will convert its operand to void, and could
+	 result in such a diagnostic being emitted.  To avoid that, convert to
+	 void ahead of time.  */
       if (t->var)
 	{
 	  tree var = t->var;
@@ -3213,7 +3219,7 @@ maybe_promote_temps (tree *stmt, void *d)
 	  if (TREE_CODE (t->init) == COND_EXPR)
 	    process_conditional (t, vlist);
 	  else
-	    finish_expr_stmt (t->init);
+	    finish_expr_stmt (convert_to_void (t->init, ICV_STATEMENT, tf_none));
 	  if (tree cleanup = cxx_maybe_build_cleanup (var, tf_warning_or_error))
 	    {
 	      tree cl = build_stmt (sloc, CLEANUP_STMT, expr_list, cleanup, var);
@@ -3232,7 +3238,7 @@ maybe_promote_temps (tree *stmt, void *d)
 	  if (TREE_CODE (t->init) == COND_EXPR)
 	    process_conditional (t, vlist);
 	  else
-	    finish_expr_stmt (t->init);
+	    finish_expr_stmt (convert_to_void (t->init, ICV_STATEMENT, tf_none));
 	  if (expr_list)
 	    {
 	      if (TREE_CODE (expr_list) != STATEMENT_LIST)
diff --git a/gcc/testsuite/g++.dg/coroutines/maybe-unused-1.C b/gcc/testsuite/g++.dg/coroutines/maybe-unused-1.C
new file mode 100644
index 000000000000..68d59d83e8eb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/coroutines/maybe-unused-1.C
@@ -0,0 +1,33 @@
+// https://gcc.gnu.org/PR116502
+#include <coroutine>
+
+struct SuspendNever {
+  bool await_ready() noexcept;
+  void await_suspend(std::coroutine_handle<>) noexcept;
+  void await_resume() noexcept;
+};
+
+struct Coroutine;
+
+struct PromiseType {
+  Coroutine get_return_object();
+  SuspendNever initial_suspend();
+  SuspendNever final_suspend() noexcept;
+  void return_void();
+  void unhandled_exception();
+};
+
+struct Coroutine {
+  using promise_type = PromiseType;
+};
+
+struct Awaiter {
+  bool await_ready();
+  void await_suspend(std::coroutine_handle<>);
+  [[nodiscard]] int& await_resume();
+};
+
+Coroutine foo()
+{
+  co_await Awaiter {}; // { dg-warning "Wunused-result" }
+}
diff --git a/gcc/testsuite/g++.dg/coroutines/pr116502.C b/gcc/testsuite/g++.dg/coroutines/pr116502.C
new file mode 100644
index 000000000000..95cc0bc8a983
--- /dev/null
+++ b/gcc/testsuite/g++.dg/coroutines/pr116502.C
@@ -0,0 +1,33 @@
+// https://gcc.gnu.org/PR116502
+#include <coroutine>
+
+struct SuspendNever {
+  bool await_ready() noexcept;
+  void await_suspend(std::coroutine_handle<>) noexcept;
+  void await_resume() noexcept;
+};
+
+struct Coroutine;
+
+struct PromiseType {
+  Coroutine get_return_object();
+  SuspendNever initial_suspend();
+  SuspendNever final_suspend() noexcept;
+  void return_void();
+  void unhandled_exception();
+};
+
+struct Coroutine {
+  using promise_type = PromiseType;
+};
+
+struct Awaiter {
+  bool await_ready();
+  void await_suspend(std::coroutine_handle<>);
+  [[nodiscard]] int& await_resume();
+};
+
+Coroutine foo()
+{
+  (void)co_await Awaiter {};
+}