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.)
