[Bug c++/96283] New: "undefined vtable" error should indicate which members are missing

[eyalroz at technion dot ac.il]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96283

            Bug ID: 96283
           Summary: "undefined vtable" error should indicate which members
                    are missing
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eyalroz at technion dot ac.il
  Target Milestone: ---

Consider the following code:

class Base {
public:
    virtual void vmethod();
};

class foo : public Base {
    int x;
    void vmethod() override;
};

int main() {
    foo f;
}

This will yield the errors (irrelevant paths snipped):

ld: prog.o: in function `Base::Base()':
<source>:1: undefined reference to `vtable for Base'
ld: prog.o: in function `foo::foo()':
<source>:6: undefined reference to `vtable for foo'

While this is true, it is a bit confusing. But even supposing I looked up what
this error means and realized what was going on, I would still need to go over
all the methods of one or two of the classes to find the one that's missing its
implementation. In this simple example that's not so difficult, but sometimes
it's quite the nuisance.

I'm assuming the compiler provides the linker with enough information to
realize which virtual methods' implementations are missing, so that the linker
can finally print an error message which methods are still missing after it has
run.

In this specific case, the linker should complain about vmethod() missing its
definition.

GodBolt: https://godbolt.org/z/9Ejn4s