> On 18 Nov 2021, at 22:13, Jason Merrill via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>
> On 11/5/21 11:46, Iain Sandoe wrote:
>> The way in which a C++20 coroutine is specified discards any value
>> that might be returned from the initial or final await expressions.
>> This PR ICE was caused by an initial await expression with an
>> await_resume () returning a reference, the function rewrite code
>> was not set up to expect this.
>> Fixed by looking through any indirection present and by explicitly
>> discarding the value, if any, returned by await_resume().
>> It does not seem useful to make a diagnostic for this, since
>> the user could define a generic awaiter that usefully returns
>> values when used in a different position from the initial (or
>> final) await expressions.
>> tested on x86_64 darwin, linux,
>> OK for master and backports?
>> thanks
>> Iain
>> Signed-off-by: Iain Sandoe <i...@sandoe.co.uk>
>> PR c++/100127
>> gcc/cp/ChangeLog:
>> * coroutines.cc (coro_rewrite_function_body): Handle initial
>> await expressions that try to produce a reference value.
>> gcc/testsuite/ChangeLog:
>> * g++.dg/coroutines/pr100127.C: New test.
>> ---
>> gcc/cp/coroutines.cc | 9 ++-
>> gcc/testsuite/g++.dg/coroutines/pr100127.C | 65 ++++++++++++++++++++++
>> 2 files changed, 73 insertions(+), 1 deletion(-)
>> create mode 100644 gcc/testsuite/g++.dg/coroutines/pr100127.C
>> diff --git a/gcc/cp/coroutines.cc b/gcc/cp/coroutines.cc
>> index 9017902e6fb..6db4b70f028 100644
>> --- a/gcc/cp/coroutines.cc
>> +++ b/gcc/cp/coroutines.cc
>> @@ -4211,9 +4211,16 @@ coro_rewrite_function_body (location_t fn_start, tree
>> fnbody, tree orig,
>> {
>> /* Build a compound expression that sets the
>> initial-await-resume-called variable true and then calls the
>> - initial suspend expression await resume. */
>> + initial suspend expression await resume.
>> + In the case that the user decides to make the initial await
>> + await_resume() return a value, we need to discard it and, it is
>> + a reference type, look past the indirection. */
>> + if (INDIRECT_REF_P (initial_await))
>> + initial_await = TREE_OPERAND (initial_await, 0);
>> tree vec = TREE_OPERAND (initial_await, 3);
>> tree aw_r = TREE_VEC_ELT (vec, 2);
>> + if (!VOID_TYPE_P (TREE_TYPE (aw_r)))
>> + aw_r = build1 (CONVERT_EXPR, void_type_node, aw_r);
>
> Is there a reason not to use convert_to_void?
no, just me still learning APIs… I’ll do a revised and check it.
Iain
>
>> tree update = build2 (MODIFY_EXPR, boolean_type_node, i_a_r_c,
>> boolean_true_node);
>> aw_r = cp_build_compound_expr (update, aw_r, tf_warning_or_error);
>> diff --git a/gcc/testsuite/g++.dg/coroutines/pr100127.C
>> b/gcc/testsuite/g++.dg/coroutines/pr100127.C
>> new file mode 100644
>> index 00000000000..374cd710077
>> --- /dev/null
>> +++ b/gcc/testsuite/g++.dg/coroutines/pr100127.C
>> @@ -0,0 +1,65 @@
>> +#ifdef __clang__
>> +#include <experimental/coroutine>
>> +namespace std {
>> + using namespace std::experimental;
>> +}
>> +#else
>> +#include <coroutine>
>> +#endif
>> +#include <optional>
>> +
>> +struct future
>> +{
>> + using value_type = int;
>> + struct promise_type;
>> + using handle_type = std::coroutine_handle<promise_type>;
>> +
>> + handle_type _coroutine;
>> +
>> + future(handle_type h) : _coroutine{h} {}
>> +
>> + ~future() noexcept{
>> + if (_coroutine) {
>> + _coroutine.destroy();
>> + }
>> + }
>> +
>> + value_type get() {
>> + auto ptr = _coroutine.promise()._value;
>> + return *ptr;
>> + }
>> +
>> + struct promise_type {
>> + std::optional<value_type> _value = std::nullopt;
>> +
>> + future get_return_object() {
>> + return future{handle_type::from_promise(*this)};
>> + }
>> + void return_value(value_type val) {
>> + _value = static_cast<value_type &&>(val);
>> + }
>> + auto initial_suspend() noexcept {
>> + class awaiter {
>> + std::optional<value_type> & value;
>> + public:
>> + explicit awaiter(std::optional<value_type> & val) noexcept
>> : value{val} {}
>> + bool await_ready() noexcept { return value.has_value(); }
>> + void await_suspend(handle_type) noexcept { }
>> + value_type & await_resume() noexcept { return *value; }
>> + };
>> +
>> + return awaiter{_value};
>> + }
>> + std::suspend_always final_suspend() noexcept {
>> + return {};
>> + }
>> + //void return_void() {}
>> + void unhandled_exception() {}
>> + };
>> +};
>> +
>> +future create_future()
>> +{ co_return 2021; }
>> +
>> +int main()
>> +{ auto f = create_future(); }
