[Bug c++/101774] New: using-declaration and typedef alter name for linkage purposes of unnamed struct

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101774

            Bug ID: 101774
           Summary: using-declaration and typedef alter name for linkage
                    purposes of unnamed struct
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: link-failure, wrong-code
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

namespace MyISA {
  typedef struct { } VecReg;
}

void foo(MyISA::VecReg &);

class A
{
  MyISA::VecReg v;
public:
  void test();
};

using MyISA::VecReg;
typedef MyISA::VecReg VecReg;

void A::test()
{
  foo(v);
}

GCC compiles this with a call to _Z3fooR6VecReg but the correct mangled name is
_Z3fooRN5MyISA6VecRegE (as emitted by Clang and EDG).

If you define the function in another TU:

namespace MyISA {
  typedef struct { } VecReg;
}

void foo(MyISA::VecReg &) { }

And try to link them, it fails:

/usr/bin/ld: /usr/lib/gcc/x86_64-redhat-linux/11/../../../../lib64/crt1.o: in
function `_start':
(.text+0x24): undefined reference to `main'
/usr/bin/ld: /tmp/cc43wYEx.o: in function `A::test()':
link.C:(.text+0x14): undefined reference to `foo(VecReg&)'
collect2: error: ld returned 1 exit status

For the wrong name to be used the using-declaration *and* the typedef must both
be present:

using MyISA::VecReg;
typedef MyISA::VecReg VecReg;

If either of those is removed, then the correct mangled name is used.