On 5/13/20 6:59 AM, Iain Sandoe wrote:
.. and now to the right list…
I came across a build failure in a folly experimental test case where,
at first, it appeared that GCC was DTRT … however, further
investigation concluded that this was a case of differing interpretations
between implementations.
It’s kinda unhelpful that the discussion appears in class elision
but it motivates the selection of the correct overload even in cases
where there’s no elision.
The (conflicting info) issue is being taken to WG21 / Core.
Yeah, let's have an xrefing note at the co_return point. not an
intercal come-from at the eliding point!
@@ -1045,25 +1044,50 @@ finish_co_return_stmt (location_t kw, tree expr)
if (!crv_meth || crv_meth == error_mark_node)
return error_mark_node;
- vec<tree, va_gc> *args = make_tree_vector_single (expr);
- co_ret_call = build_new_method_call (
- get_coroutine_promise_proxy (current_function_decl), crv_meth, &args,
- NULL_TREE, LOOKUP_NORMAL, NULL, tf_warning_or_error);
+ /* [class.copy.elision] / 3.
+ An implicitly movable entity is a variable of automatic storage
+ duration that is either a non-volatile object or an rvalue reference
+ to a non-volatile object type. For such objects in the context of
+ the co_return, the overload resolution should be carried out first
+ treating the object as an rvalue, if that fails, then we fall back
+ to regular overload resolution. */
+ tree obj = STRIP_NOPS (expr);
+ if (TREE_TYPE (obj)
+ && TYPE_REF_P (TREE_TYPE (obj))
+ && TYPE_REF_IS_RVALUE (TREE_TYPE (obj)))
+ obj = TREE_OPERAND (obj, 0);
+
+ if (TREE_CODE (obj) == PARM_DECL
+ || (VAR_P (obj)
+ && decl_storage_duration (obj) == dk_auto
+ && !TYPE_VOLATILE (TREE_TYPE (obj))))
this is wrong and insufficient. As I explained on IRC -- 'expressions of
reference type are not a thing'. We always bash them into an
indirect-ref of the referency thing.
Look at check_return_value and how it handles NRVO. There's a
treat_lvalue_as_rvalue_p call that I'll bet you can use[*], along with a
move wrapper, which maybe what you want rather than the below rvalue (expr).
a return_expr's operand is an INIT_EXPR or MODIFY_EXPR whose LHS is
DECL_RESULT(fn). So that's almost what you want here. I think the
general form is
if (treat_lval_as_rval ())
{ silently try callexpr with moved operand }
if no joy either way
{ noisily try callexpr with unmoved operand }
[*] That is insufficient, because elision also applies to objects you've
placed into the coro frame. by the std those are still automatic
storage objects, but inside the compiler they are of course
COMPONENT_REFs (or do you cons up VAR_DECLS of reference type that refer
to them?). Further, the PARM_DECL case can;t occur here -- the actor
fn's only parm is the coro frame pointer.
Either way you'll need to teach TLAR a new trick, or clone it locally
(probably the better choice).
in place of TLAR's (parm_ok && ...) I think you want
(code == COMPONENT_REF && OPERAND (0) == coro_frame)
and of course move it outside the (irrelavant) context check
does that help?
nathan
--
Nathan Sidwell
