[Bug c++/30148] Don't print "unknown" for scoped enums with no enumerators

["cvs-commit at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30148

--- Comment #2 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Andrew Burgess <aburgess@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=85c7cb3c4b70cc484ecf3d72a116503876a28f0a

commit 85c7cb3c4b70cc484ecf3d72a116503876a28f0a
Author: Andrew Burgess <aburgess@redhat.com>
Date:   Mon Feb 20 13:14:55 2023 +0000

    gdb: don't treat empty enums as flag enums

    In C++ it is possible to use an empty enum as a strong typedef.  For
    example, a user could write:

      enum class my_type : unsigned char {};

    Now my_type can be used like 'unsigned char' except the compiler will
    not allow implicit conversion too and from the native 'unsigned char'
    type.

    This is used in the standard library for things like std::byte.

    Currently, when GDB prints a value of type my_type, it looks like
    this:

      (gdb) print my_var
      $1 = (unknown: 0x4)

    Which isn't great.  This gets worse when we consider something like:

      std::vector<my_type> vec;

    When using a pretty-printer, this could look like this:

      std::vector of length 2, capacity 2 = {(unknown: 0x2), (unknown: 0x4)}

    Clearly not great.  This is described in PR gdb/30148.

    The problem here is in dwarf2/read.c, we assume all enums are flag
    enums unless we find an enumerator with a non-flag like value.
    Clearly an empty enum contains no non-flag values, so we assume the
    enum is a flag enum.

    I propose adding an extra check here; that is, an empty enum should
    never be a flag enum.

    With this the above cases look more like:

      (gdb) print my_var
      $1 = 4

    and:

      std::vector of length 2, capacity 2 = {2, 4}

    Which look much better.

    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30148

    Reviewed-By: Tom Tromey <tom@tromey.com>

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