[Bug c++/108257] New: Incorrect (non-unique) mangling of structured binding's backing variable

[Bug c++/108257] New: Incorrect (non-unique) mangling of structured binding's backing variable

[arthur.j.odwyer at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108257

            Bug ID: 108257
           Summary: Incorrect (non-unique) mangling of structured
                    binding's backing variable
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arthur.j.odwyer at gmail dot com
  Target Milestone: ---

C++20 permits structured bindings (decomposition declarations) to be `static`.
This means we can use them in inline functions, where they need to be mangled
in the context of that inline function. Clang seems to get this right; GCC
forgets to include the context.

// https://godbolt.org/z/aKanscav3
struct A { int x,y; };
inline int f() {
    static auto [a,b] = A{1,2};
    return a;
}
inline int g() {
    static auto [a,b] = A{3,4};
    return a;
}
int (*pf)() = f;
int (*pg)() = g;
int main() {
    printf("%d %d\n", f(), g());
    printf("%d %d\n", pf(), pg());
}

This program should print "1 3" twice. On GCC, it fails to assemble, because
the symbol name `_ZNDC1a1bEE` is used for both backing variables.
On Clang, the symbol names are `_ZZ1fvEDC1a1bE` and `_ZZ1gvEDC1a1bE` instead.

(Totally off-topic, it is awful that the name of the backing variable includes
all of the names of the individual bindings, concatenated. That can easily
produce humongous symbol names, and interacts unusually poorly with (UB)
macro-expansion. If anyone wants to go talk to the Itanium ABI folks and get
everyone to switch to a simple "mangling number" scheme like we have already
built for lambda closure types, that would be awesome. Notice that you already
need a "mangling number" scheme *on top* of the current design, because of
https://godbolt.org/z/cYqxzxfxe — so I'm just proposing to make the scheme
simpler, if the Itanium ABI folks are willing to change it.)

[Bug c++/108257] Incorrect (non-unique) mangling of structured binding's backing variable

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108257

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2022-12-31

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.