[Bug c++/95677] undefined reference to `(anonymous namespace)::xx'

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95677

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to liusujian from comment #2)
> (In reply to Richard Biener from comment #1)
> > It's more likely the GENERIC / cgraph output by the C++ frontend is not
> > correct
> > and works by accident without LTO.  Initial symbol table:
> > 
> > Initial Symbol table:
> > 
> > int {anonymous}::xx/3 (int {anonymous}::xx) @0x7f615d2d8180
> >   Type: variable
> >   Visibility: external
> >   References:
> >   Referring: _ZN12_GLOBAL__N_13fooEv/0 (write)
> >   Availability: not-ready
> >   Varpool flags:
> > main/2 (int main()) @0x7f615d421168
> >   Type: function definition analyzed
> >   Visibility: force_output no_reorder public
> >   Aux: @0x37a5000
> >   References: int {anonymous}::xx/1 (write)
> >   Referring:
> >   Function flags: body
> >   Called by:
> >   Calls:
> > int {anonymous}::xx/1 (int {anonymous}::xx) @0x7f615d2d8100
> >   Type: variable definition analyzed
> >   Visibility: force_output no_reorder
> >   Aux: @0x7f615d421168
> >   References:
> >   Referring: main/2 (write)
> >   Availability: not-ready
> >   Varpool flags: initialized
> > _ZN12_GLOBAL__N_13fooEv/0 (void {anonymous}::foo()) @0x7f615d421000
> >   Type: function definition analyzed
> >   Visibility: force_output no_reorder
> >   Aux: @0x7f615d2d8100
> >   References: int {anonymous}::xx/3 (write)
> >   Referring:
> >   Function flags: body
> >   Called by:
> >   Calls:
> > 
> > where you can see there are actually two 'xx' objects and the C++ FE
> > takes it up to the linker/assembler to resolve them.  But the symtab
> > code does not include such "resolving" step.
> 
> 
> In other words, C++ is currently unable to deal with this scenario ? Or any
> other problems cause the error?

The C++ frontend creates two distinct objects for 'xx' (thus it does not know
they refer to the same object).  Not sure if it is possible to exploit this
fact to create a testcase that shows this fact even without LTO.  I guess
using constexpr and fail to constexpr evaluate a function using constexpr
'xx'?  Not sure if there can be a "extern" constexpr though ;)

As said the testcase "works" without LTO because both instances mangle
to the same symbol name so the assembler/linker figure they _are_ the same
object.  Just the C++ frontend doesn't know within this TU.