Hi!
Here is an attempt to implement DR2351 - void{} - where void{} after
pack expansion is considered valid and the same thing as void().
For templates, dunno if we have some better way to check if a CONSTRUCTOR
might be empty after pack expansion. Would that only if the constructor
only contains EXPR_PACK_EXPANSION elements and nothing else, or something
else too? With the patch as is we wouldn't diagnose
template <class ...T>
void
bar (T... t)
{
void{1, t...};
}
at parsing time, only at instantiation time, even when it will always
expand to at least one CONSTRUCTOR elt.
Bootstrapped/regtested on x86_64-linux and i686-linux.
2021-10-21 Jakub Jelinek <ja...@redhat.com>
PR c++/102820
* semantics.c (finish_compound_literal): Implement DR2351 - void{}.
If type is cv void and compound_literal has no elements, return
void_node. If type is cv void and compound_literal is instantiation
dependent, handle it like other dependent compound literals.
* g++.dg/cpp0x/dr2351.C: New test.
--- gcc/cp/semantics.c.jj 2021-10-15 11:58:45.079131947 +0200
+++ gcc/cp/semantics.c 2021-10-20 17:00:38.586705600 +0200
@@ -3104,9 +3104,20 @@ finish_compound_literal (tree type, tree
if (!TYPE_OBJ_P (type))
{
- if (complain & tf_error)
- error ("compound literal of non-object type %qT", type);
- return error_mark_node;
+ /* DR2351 */
+ if (VOID_TYPE_P (type) && CONSTRUCTOR_NELTS (compound_literal) == 0)
+ return void_node;
+ else if (VOID_TYPE_P (type)
+ && processing_template_decl
+ && instantiation_dependent_expression_p (compound_literal))
+ /* If there are packs in compound_literal, it could
+ be void{} after pack expansion. */;
+ else
+ {
+ if (complain & tf_error)
+ error ("compound literal of non-object type %qT", type);
+ return error_mark_node;
+ }
}
if (template_placeholder_p (type))
--- gcc/testsuite/g++.dg/cpp0x/dr2351.C.jj 2021-10-20 17:06:02.399162937
+0200
+++ gcc/testsuite/g++.dg/cpp0x/dr2351.C 2021-10-20 17:05:54.294276511 +0200
@@ -0,0 +1,36 @@
+// DR2351
+// { dg-do compile { target c++11 } }
+
+void
+foo ()
+{
+ void{};
+ void();
+}
+
+template <class ...T>
+void
+bar (T... t)
+{
+ void{t...};
+ void(t...);
+}
+
+void
+baz ()
+{
+ bar ();
+}
+
+template <class ...T>
+void
+qux (T... t)
+{
+ void{t...}; // { dg-error "compound literal of non-object type" }
+}
+
+void
+corge ()
+{
+ qux (1, 2);
+}
Jakub
