cp+coroutines: teach convert_to_void to diagnose discarded co_awaits

co_await expressions are nearly calls to Awaitable::await_resume, and,
as such, should inherit its nodiscard.  A discarded co_await expression
should, hence, act as if its call to await_resume was discarded.

This patch teaches convert_to_void how to discard 'through' a
CO_AWAIT_EXPR. When we discard a CO_AWAIT_EXPR, we can also just discard
the await_resume() call conveniently embedded within it.  This results
in a [[nodiscard]] diagnostic that the PR noted was missing.

gcc/cp/ChangeLog:

	PR c++/110171
	* coroutines.cc (co_await_get_resume_call): New function.
	Returns the await_resume expression of a given co_await.
	* cp-tree.h (co_await_get_resume_call): New function.
	* cvt.cc (convert_to_void): Handle CO_AWAIT_EXPRs and call
	maybe_warn_nodiscard on their resume exprs.

gcc/testsuite/ChangeLog:

	PR c++/110171
	* g++.dg/coroutines/pr110171-1.C: New test.
	* g++.dg/coroutines/pr110171.C: New test.

5 files changed, 91 insertions, 1 deletions

diff --git a/gcc/cp/coroutines.cc b/gcc/cp/coroutines.cc
index 4ddd986..2b16b48 100644
--- a/gcc/cp/coroutines.cc
+++ b/gcc/cp/coroutines.cc
@@ -596,6 +596,19 @@ coro_get_destroy_function (tree decl)
   return NULL_TREE;
 }
 
+/* Given a CO_AWAIT_EXPR AWAIT_EXPR, return its resume call.  */
+
+tree*
+co_await_get_resume_call (tree await_expr)
+{
+  gcc_checking_assert (TREE_CODE (await_expr) == CO_AWAIT_EXPR);
+  tree vec = TREE_OPERAND (await_expr, 3);
+  if (!vec)
+    return nullptr;
+  return &TREE_VEC_ELT (vec, 2);
+}
+
+
 /* These functions assumes that the caller has verified that the state for
    the decl has been initialized, we try to minimize work here.  */
 
@@ -4566,7 +4579,7 @@ morph_fn_to_coro (tree orig, tree *resumer, tree *destroyer)
 	param_info *parm_i = param_uses->get (arg);
 	if (parm_i->trivial_dtor)
 	  continue;
-	add_decl_expr (parm_i->guard_var);;
+	add_decl_expr (parm_i->guard_var);
       }
   add_decl_expr (coro_promise_live);
   add_decl_expr (coro_gro_live);
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 0382467..7d50aac 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -8768,6 +8768,9 @@ extern tree coro_get_actor_function		(tree);
 extern tree coro_get_destroy_function		(tree);
 extern tree coro_get_ramp_function		(tree);
 
+extern tree* co_await_get_resume_call		(tree await_expr);
+
+
 /* contracts.cc */
 extern tree make_postcondition_variable		(cp_expr);
 extern tree make_postcondition_variable		(cp_expr, tree);
diff --git a/gcc/cp/cvt.cc b/gcc/cp/cvt.cc
index d95e01c..7b4bd8a 100644
--- a/gcc/cp/cvt.cc
+++ b/gcc/cp/cvt.cc
@@ -1502,6 +1502,14 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
 	maybe_warn_nodiscard (expr, implicit);
       break;
 
+    case CO_AWAIT_EXPR:
+      {
+	auto awr = co_await_get_resume_call (expr);
+	if (awr && *awr)
+	  *awr = convert_to_void (*awr, implicit, complain);
+	break;
+      }
+
     default:;
     }
   expr = resolve_nondeduced_context (expr, complain);
diff --git a/gcc/testsuite/g++.dg/coroutines/pr110171-1.C b/gcc/testsuite/g++.dg/coroutines/pr110171-1.C
new file mode 100644
index 0000000..d8aff58
--- /dev/null
+++ b/gcc/testsuite/g++.dg/coroutines/pr110171-1.C
@@ -0,0 +1,34 @@
+// { dg-do compile }
+#include <coroutine>
+
+struct must_check_result
+{
+    bool await_ready() { return false; }
+    void await_suspend(std::coroutine_handle<>) {}
+    [[nodiscard]] bool await_resume() { return {}; }
+};
+
+struct task {};
+
+namespace std
+{
+    template<typename... Args>
+    struct coroutine_traits<task, Args...>
+    {
+        struct promise_type
+        {
+            task get_return_object() { return {}; }
+            suspend_always initial_suspend() noexcept { return {}; }
+            suspend_always final_suspend() noexcept { return {}; }
+            void return_void() {}
+            void unhandled_exception() {}
+        };
+    };
+}
+
+task example(auto)
+{
+    co_await must_check_result(); // { dg-warning "-Wunused-result" }
+}
+
+void f() { example(1); }
diff --git a/gcc/testsuite/g++.dg/coroutines/pr110171.C b/gcc/testsuite/g++.dg/coroutines/pr110171.C
new file mode 100644
index 0000000..4b82e23
--- /dev/null
+++ b/gcc/testsuite/g++.dg/coroutines/pr110171.C
@@ -0,0 +1,32 @@
+// { dg-do compile }
+#include <coroutine>
+
+struct must_check_result
+{
+    bool await_ready() { return false; }
+    void await_suspend(std::coroutine_handle<>) {}
+    [[nodiscard]] bool await_resume() { return {}; }
+};
+
+struct task {};
+
+namespace std
+{
+    template<typename... Args>
+    struct coroutine_traits<task, Args...>
+    {
+        struct promise_type
+        {
+            task get_return_object() { return {}; }
+            suspend_always initial_suspend() noexcept { return {}; }
+            suspend_always final_suspend() noexcept { return {}; }
+            void return_void() {}
+            void unhandled_exception() {}
+        };
+    };
+}
+
+task example()
+{
+    co_await must_check_result(); // { dg-warning "-Wunused-result" }
+}