c++, coroutines: Revise promise construction/destruction.

In examining the coroutine testcases for unexpected diagnostic output
for 'Wall', I found a 'statement has no effect' warning for the promise
construction in one case.  In particular, the case is where the users
promise type has an implicit CTOR but a user-provided DTOR. Further, the
type does not actually need constructing.

In very early versions of the coroutines code we used to check
TYPE_NEEDS_CONSTRUCTING() to determine whether to attempt to build
a constructor call for the promise.  During review, it was suggested
to use type_build_ctor_call () instead.

This latter call checks the constructors in the type (both user-defined
and implicit) and returns true, amongst other cases if any of the found
CTORs are marked as deprecated.

In a number of places (for example [class.copy.ctor] / 6) the standard
says that some version of an implicit CTOR is deprecated when the user
provides a DTOR.

Thus, for this specific arrangement of promise type, type_build_ctor_call
returns true, because of (for example) a deprecated implicit copy CTOR.

We are not going to use any of the deprecated CTORs and thus will not
see warnings from this - however, since the call returned true, we have
now determined that we should attempt to build a constructor call.

Note as above, the type does not actually require construction and thus
one might expect either a NULL_TREE or error_mark_node in response to
the build_special_member_call ().  However, in practice the function
returns the original instance object instead of a call or some error.

When we add that as a statement it triggers the 'statement has no effect'
warning.

The patch here rearranges the promise construction/destruction code to
allow for the case that a DTOR is required independently of a CTOR. In
addition, we check that the return from build_special_member_call ()
has side effects before we add it as a statement.

gcc/cp/ChangeLog:

	* coroutines.cc
	(cp_coroutine_transform::build_ramp_function): Separate the
	build of promise constructor and destructor.  When evaluating
	the constructor, check that build_special_member_call returns
	an expression with side effects before adding it.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

1 files changed, 17 insertions, 13 deletions

diff --git a/gcc/cp/coroutines.cc b/gcc/cp/coroutines.cc
index e605eae..d6acf09 100644
--- a/gcc/cp/coroutines.cc
+++ b/gcc/cp/coroutines.cc
@@ -4896,16 +4896,12 @@ cp_coroutine_transform::build_ramp_function ()
   tree p = build_class_member_access_expr (deref_fp, promise_m, NULL_TREE,
 					   false, tf_warning_or_error);
 
-  tree promise_dtor = NULL_TREE;
   if (type_build_ctor_call (promise_type))
     {
-      /* Do a placement new constructor for the promise type (we never call
-	 the new operator, just the constructor on the object in place in the
-	 frame).
+      /* Construct the promise object [dcl.fct.def.coroutine] / 5.7.
 
-	 First try to find a constructor with the same parameter list as the
-	 original function (if it has params), failing that find a constructor
-	 with no parameter list.  */
+	 First try to find a constructor with an argument list comprised of
+	 the parameter copies.  */
 
       if (DECL_ARGUMENTS (orig_fn_decl))
 	{
@@ -4917,19 +4913,27 @@ cp_coroutine_transform::build_ramp_function ()
       else
 	r = NULL_TREE;
 
+      /* If that fails then the promise constructor argument list is empty.  */
       if (r == NULL_TREE || r == error_mark_node)
 	r = build_special_member_call (p, complete_ctor_identifier, NULL,
 				       promise_type, LOOKUP_NORMAL,
 				       tf_warning_or_error);
 
-      finish_expr_stmt (r);
+      /* If type_build_ctor_call() encounters deprecated implicit CTORs it will
+	 return true, and therefore we will execute this code path.  However,
+	 we might well not actually require a CTOR and under those conditions
+	 the build call above will not return a call expression, but the
+	 original instance object.  Do not attempt to add the statement unless
+	 it has side-effects.  */
+      if (r && r != error_mark_node && TREE_SIDE_EFFECTS (r))
+	finish_expr_stmt (r);
+    }
 
-      r = build_modify_expr (loc, coro_promise_live, boolean_type_node,
-			     INIT_EXPR, loc, boolean_true_node,
-			     boolean_type_node);
+  tree promise_dtor = cxx_maybe_build_cleanup (p, tf_warning_or_error);;
+  if (flag_exceptions && promise_dtor)
+    {
+      r = cp_build_init_expr (coro_promise_live, boolean_true_node);
       finish_expr_stmt (r);
-
-      promise_dtor = cxx_maybe_build_cleanup (p, tf_warning_or_error);
     }
 
   tree get_ro