[Bug c++/104918] New: Pass information to let the linker tell the user which virtual members are missing

[eyalroz1 at gmx dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104918

            Bug ID: 104918
           Summary: Pass information to let the linker tell the user which
                    virtual members are missing
           Product: gcc
           Version: 11.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eyalroz1 at gmx dot com
  Target Milestone: ---

Consider the following program:

```
struct A {
    virtual void foo() { }
};

struct B : A {
    void foo() override;
};

int main() {
    B b;
}
```

this compiles, but fails to link: https://godbolt.org/z/Mzx3c7354
```
<source>:10: undefined reference to `vtable for B'
```
which is fine, but - I'm annoyed the linker doesn't tell me which virtual
member is missing. That might be an issue with the linker, but - is foo even a
symbol in the compiled code? I tried compiling this into an object file and
using objdump (on my GNU/Linux Devuan Chimaera), and got:

a.o:     file format elf64-x86-64

SYMBOL TABLE:
0000000000000000 l    df *ABS*  0000000000000000 a.cpp
0000000000000000 l    d  .text  0000000000000000 .text
0000000000000000 l    d  .data  0000000000000000 .data
0000000000000000 l    d  .bss   0000000000000000 .bss
0000000000000000 l    d  .note.GNU-stack        0000000000000000
.note.GNU-stack
0000000000000000 l    d  .eh_frame      0000000000000000 .eh_frame
0000000000000000 l    d  .comment       0000000000000000 .comment
0000000000000000 g     F .text  0000000000000016 main
0000000000000000         *UND*  0000000000000000 vtable for B


So, no foo... and no way for the linker to be able to tell me what's missing.


I claim that GCC should expose information via the symbol table (or otherwise?)
that would let ld tell me which virtual member it's missing.