coroutines: Build pointer initializers with nullptr_node [PR107768]

The PR reports that using integer_zero_node triggers a warning for
-Wzero-as-null-pointer-constant which comes from compiler-generated code so
makes no sense to the end user.

Co-Authored-By: Iain Sandoe <iain@sandoe.co.uk>

	PR c++/107768

gcc/cp/ChangeLog:

	* coroutines.cc (coro_rewrite_function_body): Initialize pointers
	from nullptr_node.  (morph_fn_to_coro): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/pr107768.C: New test.

(cherry picked from commit 0b1d66658ecdcc3d9251641a0b902b4c73ace303)

2 files changed, 29 insertions, 3 deletions

diff --git a/gcc/cp/coroutines.cc b/gcc/cp/coroutines.cc
index 908e235..71a22b5 100644
--- a/gcc/cp/coroutines.cc
+++ b/gcc/cp/coroutines.cc
@@ -4135,7 +4135,7 @@ coro_rewrite_function_body (location_t fn_start, tree fnbody, tree orig,
   /* We will need to be able to set the resume function pointer to nullptr
      to signal that the coroutine is 'done'.  */
   tree zero_resume
-    = build1 (CONVERT_EXPR, resume_fn_ptr_type, integer_zero_node);
+    = build1 (CONVERT_EXPR, resume_fn_ptr_type, nullptr_node);
 
   /* The pointer to the destroy function.  */
   tree var = coro_build_artificial_var (fn_start, coro_destroy_fn_id,
@@ -4522,7 +4522,7 @@ morph_fn_to_coro (tree orig, tree *resumer, tree *destroyer)
   tree ramp_body = push_stmt_list ();
 
   tree zeroinit = build1_loc (fn_start, CONVERT_EXPR,
-			      coro_frame_ptr, integer_zero_node);
+			      coro_frame_ptr, nullptr_node);
   tree coro_fp = coro_build_artificial_var (fn_start, "_Coro_frameptr",
 					    coro_frame_ptr, orig, zeroinit);
   tree varlist = coro_fp;
@@ -4757,7 +4757,7 @@ morph_fn_to_coro (tree orig, tree *resumer, tree *destroyer)
 
       gcc_checking_assert (same_type_p (fn_return_type, TREE_TYPE (grooaf)));
       tree if_stmt = begin_if_stmt ();
-      tree cond = build1 (CONVERT_EXPR, coro_frame_ptr, integer_zero_node);
+      tree cond = build1 (CONVERT_EXPR, coro_frame_ptr, nullptr_node);
       cond = build2 (EQ_EXPR, boolean_type_node, coro_fp, cond);
       finish_if_stmt_cond (cond, if_stmt);
       if (VOID_TYPE_P (fn_return_type))
diff --git a/gcc/testsuite/g++.dg/coroutines/pr107768.C b/gcc/testsuite/g++.dg/coroutines/pr107768.C
new file mode 100644
index 0000000..22d7074
--- /dev/null
+++ b/gcc/testsuite/g++.dg/coroutines/pr107768.C
@@ -0,0 +1,26 @@
+//  { dg-additional-options "-Wzero-as-null-pointer-constant -fsyntax-only" }
+
+#include <coroutine>
+ 
+struct task
+{
+    struct promise_type
+    {
+        task get_return_object() { return {}; }
+        std::suspend_never initial_suspend() { return {}; }
+        std::suspend_never final_suspend() noexcept { return {}; }
+        void return_void() {}
+        void unhandled_exception() {}
+    };
+};
+ 
+task resuming_on_new_thread(void)
+{
+    struct awaitable
+    {
+        bool await_ready() { return false; }
+        void await_suspend(std::coroutine_handle<> h)         {         }
+        void await_resume() {}
+    };
+    co_await awaitable{};
+}