[Bug 305460]

[Eyalroz1]

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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https://bugs.launchpad.net/bugs/305460

Title:
  [PR42540] c++ error message [vtable undefined] is unhelpful

Status in gcc:
  In Progress
Status in gcc-defaults package in Ubuntu:
  Confirmed

Bug description:
  I've had response to g++ crash reports, so maybe you'd also like to
  know about this confusing error message I found.

  Thanks!
  Alan

  This is on an up-to-date install of "Ubuntu 8.04.1".

  apt-cache policy gcc
  gcc:
    Installed: 4:4.2.3-1ubuntu6
    Candidate: 4:4.2.3-1ubuntu6
    Version table:
   *** 4:4.2.3-1ubuntu6 0
          500 http://gb.archive.ubuntu.com hardy-updates/main Packages
          100 /var/lib/dpkg/status
       4:4.2.3-1ubuntu3 0
          500 http://gb.archive.ubuntu.com hardy/main Packages


  Minimal test program:

  class A {
      A();

      virtual void B();
  };

  A::A() {}
  /* Whoops, I forgot to define A::B() */

  Actual error message:

  /usr/lib/gcc/x86_64-linux-gnu/4.2.4/../../../../lib/crt1.o: In function `_start':
  (.text+0x20): undefined reference to `main'
  /tmp/cc8P2uDq.o: In function `A::A()':
  test.cpp:(.text+0x9): undefined reference to `vtable for A'
  /tmp/cc8P2uDq.o: In function `A::A()':
  test.cpp:(.text+0x1f): undefined reference to `vtable for A'
  collect2: ld returned 1 exit status

  Expected error message:

  Something more like "undefined reference to A::B()"

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