[Bug 305460]

[lichray]

The example in this bug is not as severe and frequent as the one in bug
44841.  Users are more likely to encounter this issue when adding a new
API to existing interface or type erasure.

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.

The issue has not been fixed for ten years.  It is a shame rather than
some arcane knowledge that worth to be proud of.

MSVC has no such issue.  The error message reads as "undefined reference
to Class::that_virtual_function."

Some possible fixes:

1. Adjust the error message to say, "The first non-inline, non-pure function may not have a definition in <Class>."
2. Add extra information to name the key function, and pass it to the linker, generate an error message to match MSVC's quality.

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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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