[Bug other/42540] c++ error message [vtable undefined] is unhelpful

["eyalroz1 at gmx dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=42540

--- Comment #16 from Eyal Rozenberg <eyalroz1 at gmx dot com> ---
Some comments following my recent dupe...

(In reply to Andrew Pinski from comment #1)
> I don't know if there is anything there could be done here since the linker
> is what is producing the error.

The compiler could store information in the compiled object listing the virtual
members for which no implementation was found, due to which reason the vtable
was not defined already. In this specific example, storing the name of "A::B()"
somewhere. If that information is available, we could then petition linker
authors to use it and print the missing virtual members in the error message.

(In reply to  Richard Earnshaw from comment #5)
> As suggested, there's no bug in the compiler here

Not passing sufficient information to the linker is a "bug", or at least - a
missing feature.

(In reply to Zhihao Yuan from comment #11)
> 2. Add extra information to name the key function, and pass it to the linker, generate an error message to match MSVC's quality.

I'm not quite sure what a key function is, but it sounds like my suggestion is
similar to this one. So, I support your suggestion (2.)

> Calling this a "well-known issue" is irresponsible.  The issue significantly
> increases the bar to learners to consume and accept new paradigms in the
> language.

Not just learners. If you have a large class with many methods, whose
implementation is spread across many files, it can take quite a bit of time to
figure out which method implementation is missing.

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