[Bug c++/110349] [C++26] P2169R4 - Placeholder variables with no name

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110349

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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  Attachment #55725|0                           |1
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--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 55757
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55757&action=edit
gcc14-pr110349-wip.patch

Thanks, made some progress with that.
I wonder if the paper isn't incomplete though.
My understanding of the intent is that say
void foo () { auto a = [_ = 1, _ = 2] () {}; }
is valid, similarly
struct S { int _ = 1; int _ = 2; };
(and it seems the clang implementation allows that),
but we still have
https://eel.is/c++draft/expr.prim.lambda.capture#2
"Ignoring appearances in initializers of init-captures, an identifier or this
shall not appear more than once in a lambda-capture."
and
https://eel.is/c++draft/class.mem#general-5
"A member shall not be declared twice in the member-specification, except that"
and nothing mentioning the name-independent exception in either case.
Another thing is we have that
https://eel.is/c++draft/basic.scope.block#2
spot which has been changed by the paper, so
void baz (int _) { int _ = 1; }
void qux () { if (int _ = 2) { int _ = 3; } }
etc. cases are valid which have been invalid before, but the important question
is if ++_; is allowed after 1; and/or 3;
https://eel.is/c++draft/basic.scope.scope#6
has the note:
"An id-expression that names a unique name-independent declaration is usable
until an additional declaration of the same name is introduced in the same
scope ([basic.lookup.general])."
but does that apply here?
https://eel.is/c++draft/basic.scope.block#2 seems to talk about the behavior
in the same scope, but aren't the scopes different here?
Seems clang rejects the baz case with ++_; added after 1; (and the WIP patch
does too), but doesn't reject ++_; added after 3; (while the WIP patch does).