coroutines: Add a cleanup expression for g-r-o when needed [PR95477].

The PR reports that we fail to destroy the object initially created from
the get-return-object call.  Fixed by adding a cleanup when the DTOR is
non-trivial.  In addition, to meet the specific wording that the call to
get_return_object creates the glvalue for the return, we must construct
that in-place in the return object to avoid a second copy/move CTOR.

gcc/cp/ChangeLog:

	PR c++/95477
	* coroutines.cc (morph_fn_to_coro): Apply a cleanup to
	the get return object when the DTOR is non-trivial.

gcc/testsuite/ChangeLog:

	PR c++/95477
	* g++.dg/coroutines/pr95477.C: New test.
	* g++.dg/coroutines/void-gro-non-class-coro.C: New test.

(cherry picked from commit 788b962aa00959e861b45767c5c88ec41ca30c21)

gcc/cp/coroutines.cc

@@ -4288,13 +4288,36 @@ morph_fn_to_coro (tree orig, tree *resumer, tree *destroyer)
   tree gro_type = TREE_TYPE (get_ro);
   bool gro_is_void_p = VOID_TYPE_P (gro_type);
 
-  tree gro, gro_bind_vars = NULL_TREE;
+  tree gro = NULL_TREE;
+  tree gro_bind_vars = NULL_TREE;
+  tree gro_cleanup_stmt = NULL_TREE;
   /* We have to sequence the call to get_return_object before initial
      suspend.  */
   if (gro_is_void_p)
-    finish_expr_stmt (get_ro);
+    r = get_ro;
+  else if (same_type_p (gro_type, fn_return_type))
+    {
+     /* [dcl.fct.def.coroutine] / 7
+	The expression promise.get_return_object() is used to initialize the
+	glvalue result or... (see below)
+	Construct the return result directly.  */
+      if (TYPE_NEEDS_CONSTRUCTING (gro_type))
+	{
+	  vec<tree, va_gc> *arg = make_tree_vector_single (get_ro);
+	  r = build_special_member_call (DECL_RESULT (orig),
+					 complete_ctor_identifier,
+					 &arg, gro_type, LOOKUP_NORMAL,
+					 tf_warning_or_error);
+	  release_tree_vector (arg);
+	}
+      else
+	r = build2_loc (fn_start, INIT_EXPR, gro_type,
+			DECL_RESULT (orig), get_ro);
+    }
   else
     {
+      /* ... or ... Construct an object that will be used as the single
+	param to the CTOR for the return object.  */
       gro = build_lang_decl (VAR_DECL, get_identifier ("coro.gro"), gro_type);
       DECL_CONTEXT (gro) = current_scope ();
       add_decl_expr (gro);
@@ -4309,8 +4332,21 @@ morph_fn_to_coro (tree orig, tree *resumer, tree *destroyer)
 	}
       else
 	r = build2_loc (fn_start, INIT_EXPR, gro_type, gro, get_ro);
-      finish_expr_stmt (r);
+      /* The constructed object might require a cleanup.  */
+      if (TYPE_HAS_NONTRIVIAL_DESTRUCTOR (gro_type))
+	{
+	  tree cleanup
+	    = build_special_member_call (gro, complete_dtor_identifier,
+					 NULL, gro_type, LOOKUP_NORMAL,
+					 tf_warning_or_error);
+	  gro_cleanup_stmt = build_stmt (input_location, CLEANUP_STMT, NULL,
+					 cleanup, gro);
+	}
     }
+  finish_expr_stmt (r);
+
+  if (gro_cleanup_stmt)
+    CLEANUP_BODY (gro_cleanup_stmt) = push_stmt_list ();
 
   /* Initialize the resume_idx_name to 0, meaning "not started".  */
   tree resume_idx_m
@@ -4352,21 +4388,20 @@ morph_fn_to_coro (tree orig, tree *resumer, tree *destroyer)
      promise was constructed.  We now supply a reference to that var,
      either as the return value (if it's the same type) or to the CTOR
      for an object of the return type.  */
-  if (gro_is_void_p)
-    r = NULL_TREE;
-  else
-    r = rvalue (gro);
 
-  if (!same_type_p (gro_type, fn_return_type))
+  if (same_type_p (gro_type, fn_return_type))
+    r = gro_is_void_p ? NULL_TREE : DECL_RESULT (orig);
+  else
     {
-      /* The return object is , even if the gro is void.  */
       if (CLASS_TYPE_P (fn_return_type))
 	{
+	  /* For class type return objects, we can attempt to construct,
+	     even if the gro is void.  */
 	  vec<tree, va_gc> *args = NULL;
 	  vec<tree, va_gc> **arglist = NULL;
 	  if (!gro_is_void_p)
 	    {
-	      args = make_tree_vector_single (r);
+	      args = make_tree_vector_single (rvalue (gro));
 	      arglist = &args;
 	    }
 	  r = build_special_member_call (NULL_TREE,
@@ -4377,12 +4412,27 @@ morph_fn_to_coro (tree orig, tree *resumer, tree *destroyer)
 	  if (args)
 	    release_tree_vector (args);
 	}
-      else /* ??? suppose we have non-class return and void gro?  */
-	r = build1_loc (input_location, CONVERT_EXPR, fn_return_type, r);
+      else if (gro_is_void_p)
+	{
+	  /* We can't initialize a non-class return value from void.  */
+	  error_at (input_location, "cannot initialize a return object of type"
+		    " %qT with an rvalue of type %<void%>", fn_return_type);
+	  r = error_mark_node;
+	}
+      else
+	r = build1_loc (input_location, CONVERT_EXPR,
+			fn_return_type, rvalue (gro));
     }
 
   finish_return_stmt (r);
 
+  if (gro_cleanup_stmt)
+    {
+      CLEANUP_BODY (gro_cleanup_stmt)
+	= pop_stmt_list (CLEANUP_BODY (gro_cleanup_stmt));
+      add_stmt (gro_cleanup_stmt);
+    }
+
   /* Finish up the ramp function.  */
   BIND_EXPR_VARS (gro_context_bind) = gro_bind_vars;
   BIND_EXPR_BODY (gro_context_bind) = pop_stmt_list (gro_context_body);

gcc/testsuite/g++.dg/coroutines/pr95477.C

+//  { dg-do run }
+
+#include "coro.h"
+
+struct simple {
+  static inline int alive = 0;
+  simple() { ++alive; }
+  simple(simple&&) { ++alive; }
+  ~simple() { --alive; }
+
+  struct promise_type {
+    simple get_return_object() { return simple{}; }
+    void return_void() {}
+    void unhandled_exception() {}
+    auto initial_suspend() noexcept { return coro::suspend_never{}; }
+    auto final_suspend() noexcept { return coro::suspend_never{}; }
+  };
+};
+
+simple
+f()
+{
+  co_return;
+}
+
+int main() {
+  {
+    f();
+  }
+
+  if (simple::alive != 0)
+   {
+     PRINTF ("something wrong with dtors: %d\n", simple::alive);
+     abort ();
+   }
+  return 0;
+}

gcc/testsuite/g++.dg/coroutines/void-gro-non-class-coro.C

+// Test handling of the case where we have a void g-r-o and a non-void
+// and non-class-type ramp return.
+
+#include "coro.h"
+
+int g_promise = -1;
+
+template<typename R, typename HandleRef, typename ...T>
+struct std::coroutine_traits<R, HandleRef, T...> {
+    struct promise_type {
+        promise_type (HandleRef h, T ...args)
+        { h = std::coroutine_handle<promise_type>::from_promise (*this);
+          PRINT ("Created Promise");
+          g_promise = 1;
+        }
+	~promise_type () { PRINT ("Destroyed Promise"); g_promise = 0;}
+        void get_return_object() {}
+
+        auto initial_suspend() {
+          return std::suspend_always{};
+         }
+        auto final_suspend() { return std::suspend_never{}; }
+
+        void return_void() {}
+        void unhandled_exception() {}
+    };
+};
+
+int
+my_coro (std::coroutine_handle<>& h)
+{
+  PRINT ("coro1: about to return");
+  co_return;
+} // { dg-error {cannot initialize a return object of type 'int' with an rvalue of type 'void'} }
+
+int main ()
+{
+  std::coroutine_handle<> h;
+  int t = my_coro (h);
+
+  if (h.done())
+    {
+      PRINT ("main: apparently was already done...");
+      abort ();
+    }
+
+  // initial suspend.
+  h.resume ();
+
+  // The coro should have self-destructed.
+  if (g_promise)
+    {
+      PRINT ("main: apparently we did not complete...");
+      abort ();
+    }
+
+  PRINT ("main: returning");
+  return 0;
+}