[Bug c++/116502] New: [15 Regression] -Wunused-result warning is not supressed if coroutine awaiter returns a reference

[Bug c++/116502] New: [15 Regression] -Wunused-result warning is not supressed if coroutine awaiter returns a reference

[daklishch at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116502

            Bug ID: 116502
           Summary: [15 Regression] -Wunused-result warning is not
                    supressed if coroutine awaiter returns a reference
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Keywords: c++-coroutines
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: daklishch at gmail dot com
                CC: arsen at gcc dot gnu.org
  Target Milestone: ---

After 2664c1bf83855b79d8c43772e71b87ed4f09c174 (cp+coroutines: teach
convert_to_void to diagnose discarded co_awaits), the following code produces
-Wunused-result warning, while the caller clearly wants to suppress it by
casting the co_await result to void.

```
$ cat ../test.cpp
#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();
};

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 {};
}

$ ~/gcc-15-trunk/bin/g++ ../test.cpp -c -std=c++20 -fno-exceptions
-Wunused-result -o /dev/null
../test.cpp: In function ‘Coroutine foo()’:
../test.cpp:30:29: warning: ignoring return value of ‘int&
Awaiter::await_resume()’, declared with attribute ‘nodiscard’ [-Wunused-result]
   30 |     (void)co_await Awaiter {};
      |                             ^
../test.cpp:25:24: note: declared here
   25 |     [[nodiscard]] int& await_resume();
      |                        ^~~~~~~~~~~~
```

The warning is correctly suppressed unless Awaiter::await_resume returns a
reference.

[Bug c++/116502] [15 Regression] -Wunused-result warning cannot be suppressed if coroutine awaiter returns a reference after r15-2318-g2664c1bf83855b

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116502

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[15 Regression]             |[15 Regression]
                   |-Wunused-result warning     |-Wunused-result warning
                   |cannot be suppressed if     |cannot be suppressed if
                   |coroutine awaiter returns a |coroutine awaiter returns a
                   |reference                   |reference after
                   |                            |r15-2318-g2664c1bf83855b
   Target Milestone|---                         |15.0

[Bug c++/116502] [15 Regression] -Wunused-result warning cannot be suppressed if coroutine awaiter returns a reference after r15-2318-g2664c1bf83855b

[arsen at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116502

Arsen Arsenović <arsen at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2024-08-27

--- Comment #1 from Arsen Arsenović <arsen at gcc dot gnu.org> ---
thanks for the report!

candidate patchlet:

modified   gcc/cp/coroutines.cc
@@ -3423,7 +3423,14 @@ 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;
@@ -3433,7 +3440,7 @@ 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 (build1 (CONVERT_EXPR, void_type_node, t->init));
          if (tree cleanup = cxx_maybe_build_cleanup (var,
tf_warning_or_error))
            {
              tree cl = build_stmt (sloc, CLEANUP_STMT, expr_list, cleanup,
var);
@@ -3452,7 +3459,7 @@ 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 (build1 (CONVERT_EXPR, void_type_node, t->init));
          if (expr_list)
            {
              if (TREE_CODE (expr_list) != STATEMENT_LIST)

[Bug c++/116502] [15 Regression] -Wunused-result warning cannot be suppressed if coroutine awaiter returns a reference after r15-2318-g2664c1bf83855b

[daklishch at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116502

--- Comment #2 from Dan Klishch <daklishch at gmail dot com> ---
Can confirm that the patch fixes both the minified reproducer from the bug
report and also the original reproducer (TestAsyncTestStreams from SerenityOS).

[Bug c++/116502] [15 Regression] -Wunused-result warning cannot be suppressed if coroutine awaiter returns a reference after r15-2318-g2664c1bf83855b

[arsen at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116502

Arsen Arsenović <arsen at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |arsen at gcc dot gnu.org

--- Comment #3 from Arsen Arsenović <arsen at gcc dot gnu.org> ---
mine, patch sent a few days ago (still needs some work)