Re: [PATCH] gdb: don't treat empty enums as flag enums

[Andrew Burgess <aburgess@redhat.com>]

Andreas Schwab <schwab@suse.de> writes:

> On Feb 20 2023, Andrew Burgess via Gdb-patches wrote:
>
>> 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 = (unknown: 0x4)
>
> I guess this should look different from the output above?

Oops.  Yeah, I updated the std::vector example below, but this one
should be:

  (gdb) print my_var
  $1 = 0x4

I've updated the commit message locally.

Thanks,
Andrew