On 10/21/21 04:42, Jakub Jelinek wrote:
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?
I think that's the only case. For template args there's the pack_expansion_args_count function, but I don't think there's anything similar for constructor elts; please feel free to add it.
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, treeif (!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
