On 7/23/24 7:41 PM, Arsen Arsenović wrote:
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.
CO_AWAIT_EXPR trees do conveniently contain the expression for calling
await_resume in them, so we can discard it.
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.
---
This patch teaches convert_to_void how to discard 'through' a
CO_AWAIT_EXPR. CO_AWAIT_EXPR nodes (most of the time) already contain
their relevant await_resume() call embedded within them, so, when we
discard a CO_AWAIT_EXPR, we can also just discard the await_resume()
call embedded within it. This results in a [[nodiscard]] diagnostic
that the PR noted was missing.
Again you have two different versions of the patch rationale?
As with the previous patch, regression-tested on x86_64-pc-linux-gnu.
OK for trunk?
TIA.
gcc/cp/coroutines.cc | 13 ++++++++
gcc/cp/cp-tree.h | 3 ++
gcc/cp/cvt.cc | 8 +++++
gcc/testsuite/g++.dg/coroutines/pr110171-1.C | 34 ++++++++++++++++++++
gcc/testsuite/g++.dg/coroutines/pr110171.C | 32 ++++++++++++++++++
5 files changed, 90 insertions(+)
create mode 100644 gcc/testsuite/g++.dg/coroutines/pr110171-1.C
create mode 100644 gcc/testsuite/g++.dg/coroutines/pr110171.C
diff --git a/gcc/cp/coroutines.cc b/gcc/cp/coroutines.cc
index fb8f24e6c61..05486c2fb19 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*
Why return tree* rather than 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. */
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 856699de82f..c9ae8950bd1 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -8763,6 +8763,9 @@ 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 d95e01c118c..7b4bd8a9dc4 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 00000000000..d8aff582487
--- /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 00000000000..4b82e23656c
--- /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" }
+}
