Hi!
The following patch attempts to implement this paper by dropping the
coroutine diagnostics about both return_void and return_value (which
apparently still has various problems, so perhaps we should fix those on
release branches:
1) the error is guarded on !coro_info->coro_co_return_error_emitted
but the inform calls are not, so the first time we run into this,
we emit an error and 2 informs, the next time we run there we just
confuse the user by emitting further 2 informs without emitting
an error. All the diagnostic handling should be guarded on
!coro_info->coro_co_return_error_emitted I guess and only the
return shouldn't
2) if promise_type has e.g.
void return_value(int x) { }
void return_void() {}
void return_void(int) {}
we ICE on it or emit bogus location, because has_ret_void in
that case is also OVERLOAD, not a decl). Also, I've added complain
argument to some functions, because
https://eel.is/c++draft/stmt.return.coroutine#3
now says If overload resolution for p.return_void() succeeds, ...
So, we shouldn't error in any way if p.return_void() overload resolution
fails. I admit I'm not sure if passing tf_none exactly to the functions
I've passed it down to (lookup_member and build_new_method_call or
build_class_member_access_expr) achieves that, or if those functions
can fail with further errors after overload resolution. If the
latter, what exactly should be called with tf_none to determine whether
overload resolution succeeds?
So far lightly tested (on coroutines/* and affected tests), ok for trunk
if it passes full bootstrap/regtest? And what to do about the above
mentioned bugs in the code being removed? Shall I address those first
and then send this patch to remove it afterwards (and just backport that
first patch and not the second)?
2026-10-01 Jakub Jelinek <[email protected]>
P3950R1
PR c++/125830
* c-cppbuiltin.cc (c_cpp_builtins): Bump __cpp_impl_coroutine
from 201902L to 202606L.
* coroutines.cc: Implement C++29 P3950R1 - return_value & return_void
Are Not Mutually Exclusive paper as DR.
(struct coroutine_info): Remove coro_co_return_error_emitted member.
(coro_promise_type_found_p): Remove diagnostics about both return_void
and return_value successful lookup.
(lookup_promise_method): Add complain argument, pass it down to
lookup_member.
(coro_build_promise_expression): Add complain argument, pass it down
to lookup_promise_method, build_new_method_call and
build_class_member_access_expr.
(get_coroutine_return_void_expr): Pass tf_none to
coro_build_promise_expression.
* g++.dg/DRs/dr2581-2.C (__cpp_impl_coroutine): Redefine to 202606L
rather than 201902L.
* g++.dg/coroutines/pr105301.C: Change dg-error to dg-bogus.
* g++.dg/coroutines/coro-pre-proc.C: Expect different value of
__cpp_impl_coroutine.
* g++.dg/coroutines/pr97438.C: Change dg-error to dg-bogus.
* g++.dg/coroutines/p3950r1.C: New test.
--- a/gcc/c-family/c-cppbuiltin.cc 2026-09-10 16:56:41.686638893 +0200
+++ b/gcc/c-family/c-cppbuiltin.cc 2026-10-01 12:42:04.068512619 +0200
@@ -1152,7 +1152,7 @@ c_cpp_builtins (cpp_reader *pfile)
else if (cxx_dialect >= cxx20)
cpp_warn (pfile, "__cpp_modules");
if (flag_coroutines)
- cpp_define (pfile, "__cpp_impl_coroutine=201902L"); /* n4861, DIS */
+ cpp_define (pfile, "__cpp_impl_coroutine=202606L"); /* n4861, DIS */
else if (cxx_dialect >= cxx20)
cpp_warn (pfile, "__cpp_impl_coroutine");
if (flag_tm)
--- a/gcc/cp/coroutines.cc 2026-09-25 17:52:15.184532722 +0200
+++ b/gcc/cp/coroutines.cc 2026-10-01 13:21:12.311479268 +0200
@@ -298,7 +298,6 @@ struct GTY((for_user)) coroutine_info
/* Flags to avoid repeated errors for per-function issues. */
bool coro_ret_type_error_emitted;
bool coro_promise_error_emitted;
- bool coro_co_return_error_emitted;
};
struct coroutine_info_hasher : ggc_ptr_hash<coroutine_info>
@@ -825,37 +824,6 @@ coro_promise_type_found_p (tree fndecl,
return false;
}
- /* Test for errors in the promise type that can be determined now. */
- tree has_ret_void = lookup_member (coro_info->promise_type,
- coro_return_void_identifier,
- /*protect=*/1, /*want_type=*/0,
- tf_none);
- tree has_ret_val = lookup_member (coro_info->promise_type,
- coro_return_value_identifier,
- /*protect=*/1, /*want_type=*/0,
- tf_none);
- if (has_ret_void && has_ret_val)
- {
- auto_diagnostic_group d;
- location_t ploc = DECL_SOURCE_LOCATION (fndecl);
- if (!coro_info->coro_co_return_error_emitted)
- error_at (ploc, "the coroutine promise type %qT declares both"
- " %<return_value%> and %<return_void%>",
- coro_info->promise_type);
- inform (DECL_SOURCE_LOCATION (BASELINK_FUNCTIONS (has_ret_void)),
- "%<return_void%> declared here");
- has_ret_val = BASELINK_FUNCTIONS (has_ret_val);
- const char *message = "%<return_value%> declared here";
- if (TREE_CODE (has_ret_val) == OVERLOAD)
- {
- has_ret_val = OVL_FIRST (has_ret_val);
- message = "%<return_value%> first declared here";
- }
- inform (DECL_SOURCE_LOCATION (has_ret_val), message);
- coro_info->coro_co_return_error_emitted = true;
- return false;
- }
-
/* Try to find the handle type for the promise. */
tree handle_type
= instantiate_coro_handle_for_promise_type (loc,
coro_info->promise_type);
@@ -1028,12 +996,13 @@ get_coroutine_from_address (tree decl)
static tree
lookup_promise_method (tree fndecl, tree member_id, location_t loc,
- bool musthave)
+ bool musthave,
+ tsubst_flags_t complain = tf_warning_or_error)
{
tree promise = get_coroutine_promise_type (fndecl);
tree pm_memb
= lookup_member (promise, member_id,
- /*protect=*/1, /*want_type=*/0, tf_warning_or_error);
+ /*protect=*/1, /*want_type=*/0, complain);
if (musthave && pm_memb == NULL_TREE)
{
error_at (loc, "no member named %qE in %qT", member_id, promise);
@@ -1051,9 +1020,10 @@ lookup_promise_method (tree fndecl, tree
static tree
coro_build_promise_expression (tree fn, tree promise_obj, tree member_id,
location_t loc, vec<tree, va_gc> **args,
- bool musthave)
+ bool musthave,
+ tsubst_flags_t complain = tf_warning_or_error)
{
- tree meth = lookup_promise_method (fn, member_id, loc, musthave);
+ tree meth = lookup_promise_method (fn, member_id, loc, musthave, complain);
if (meth == error_mark_node)
return error_mark_node;
@@ -1067,11 +1037,11 @@ coro_build_promise_expression (tree fn,
tree expr;
if (BASELINK_P (meth))
expr = build_new_method_call (promise, meth, args, NULL_TREE,
- LOOKUP_NORMAL, NULL, tf_warning_or_error);
+ LOOKUP_NORMAL, NULL, complain);
else
{
expr = build_class_member_access_expr (promise, meth, NULL_TREE,
- true, tf_warning_or_error);
+ true, complain);
vec<tree, va_gc> *real_args;
if (!args)
real_args = make_tree_vector ();
@@ -1094,7 +1064,7 @@ get_coroutine_return_void_expr (tree dec
info->return_void
= coro_build_promise_expression (current_function_decl, NULL,
coro_return_void_identifier,
- loc, NULL, musthave);
+ loc, NULL, musthave, tf_none);
/* Don't return an error if it's an optional call. */
if (!musthave && info->return_void == error_mark_node)
return NULL_TREE;
--- a/gcc/testsuite/g++.dg/DRs/dr2581-2.C 2026-08-06 11:48:09.058480111
+0200
+++ b/gcc/testsuite/g++.dg/DRs/dr2581-2.C 2026-10-01 12:44:13.876768629
+0200
@@ -61,7 +61,7 @@
#define __cpp_hex_float 201603L // { dg-error
"'__cpp_hex_float' redefined" "" { target c++20 } }
#define __cpp_if_consteval 202106L // { dg-error
"'__cpp_if_consteval' redefined" "" { target c++23 } }
#define __cpp_if_constexpr 201606L // { dg-error
"'__cpp_if_constexpr' redefined" "" { target c++20 } }
-#define __cpp_impl_coroutine 201902L // { dg-error
"'__cpp_impl_coroutine' redefined" "" { target c++20 } }
+#define __cpp_impl_coroutine 202606L // { dg-error
"'__cpp_impl_coroutine' redefined" "" { target c++20 } }
#define __cpp_impl_destroying_delete 201806L // { dg-error
"'__cpp_impl_destroying_delete' redefined" "" { target c++20 } }
#define __cpp_impl_three_way_comparison 201907L // { dg-error
"'__cpp_impl_three_way_comparison' redefined" "" { target c++20 } }
#define __cpp_impl_reflection 202603L // { dg-error
"'__cpp_impl_reflection' redefined" "" { target c++26 } }
--- a/gcc/testsuite/g++.dg/coroutines/pr105301.C 2026-03-27
10:17:15.380310407 +0100
+++ b/gcc/testsuite/g++.dg/coroutines/pr105301.C 2026-10-01
12:40:29.792779226 +0200
@@ -46,7 +46,7 @@ struct bad_promise_6 {
};
coro<bad_promise_6>
-bad_implicit_return() // { dg-error {.aka 'bad_promise_6'. declares both
'return_value' and 'return_void'} }
+bad_implicit_return() // { dg-bogus {.aka 'bad_promise_6'. declares both
'return_value' and 'return_void'} }
{
co_await a;
}
--- a/gcc/testsuite/g++.dg/coroutines/coro-pre-proc.C 2026-03-27
10:17:15.379310423 +0100
+++ b/gcc/testsuite/g++.dg/coroutines/coro-pre-proc.C 2026-10-01
12:44:29.900553347 +0200
@@ -4,6 +4,6 @@
#error "coroutines should engaged."
#endif
-#if __cpp_impl_coroutine != 201902L
+#if __cpp_impl_coroutine != 202606L
#error "coroutine version out of sync."
#endif
--- a/gcc/testsuite/g++.dg/coroutines/pr97438.C 2026-03-27 10:17:15.384310342
+0100
+++ b/gcc/testsuite/g++.dg/coroutines/pr97438.C 2026-10-01 12:46:40.857793918
+0200
@@ -25,6 +25,6 @@ public:
}
dummy_coroutine
-foo() { // { dg-error {the coroutine promise type
'std::__n4861::coroutine_traits<dummy_coroutine>::promise_type' declares both
'return_value' and 'return_void'} }
+foo() { // { dg-bogus {the coroutine promise type
'std::__n4861::coroutine_traits<dummy_coroutine>::promise_type' declares both
'return_value' and 'return_void'} }
co_return 17;
}
--- a/gcc/testsuite/g++.dg/coroutines/p3950r1.C 2026-10-01 12:52:43.323958890
+0200
+++ b/gcc/testsuite/g++.dg/coroutines/p3950r1.C 2026-10-01 13:06:52.331832286
+0200
@@ -0,0 +1,74 @@
+// C++29 P3950R1 - return_value & return_void Are Not Mutually Exclusive
+
+#include <coroutine>
+#include <type_traits>
+
+struct dummy_coroutine1 {};
+template <typename T>
+struct dummy_coroutine2 {};
+
+namespace std {
+
+template<>
+class coroutine_traits<::dummy_coroutine1> {
+public:
+ struct promise_type {
+ void return_value(int x) {}
+ void return_value(double x) {}
+ void return_void() {}
+ std::suspend_never initial_suspend() noexcept { return {}; }
+ std::suspend_never final_suspend() noexcept { return {}; }
+ dummy_coroutine1 get_return_object() { return {}; }
+ void unhandled_exception() {}
+ };
+};
+
+template<typename T>
+class coroutine_traits<::dummy_coroutine2<T>> {
+public:
+ struct promise_type {
+ template<typename U = T>
+ void return_value(U x) requires std::is_same_v<U, T> {}
+ void return_void() requires std::is_same_v<void, T> {}
+ std::suspend_never initial_suspend() noexcept { return {}; }
+ std::suspend_never final_suspend() noexcept { return {}; }
+ dummy_coroutine2<T> get_return_object() { return {}; }
+ void unhandled_exception() {}
+ };
+};
+
+}
+
+int x;
+
+dummy_coroutine1
+foo ()
+{
+ if (x == 1)
+ co_return 17;
+ else if (x == 2)
+ co_return 17.0;
+ else if (x == 3)
+ co_return;
+}
+
+dummy_coroutine2<void>
+bar ()
+{
+ if (x == 4)
+ co_return;
+}
+
+dummy_coroutine2<int>
+baz ()
+{
+ if (x == 5)
+ co_return 42;
+}
+
+dummy_coroutine2<double>
+qux ()
+{
+ if (x == 6)
+ co_return 42.0;
+}
Jakub
