[Bug gdb/28480] pretty printing getting confused

["jwakely.gcc at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28480

Jonathan Wakely <jwakely.gcc at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=103086

--- Comment #11 from Jonathan Wakely <jwakely.gcc at gmail dot com> ---
I've created https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103086 for this, with
a more detailed explanation.

I think GDB's new behaviour is probably better, and the libstdc++ printers
should be fixed.

However, Bruno did suggest that when tuple_member['_M_head_impl'] is ambiguous
it should give an error, not silently return either the first or last matching
member.

For example, in this code:

struct X
{
  int i = 0x1;
};

struct Y
{
  char i = 'Y';
};

struct Z : X, Y { };

int main()
{
  Z z;
  return 0;
}

GDB 10 will happily print z.i

$ gdb -ex start -ex n -ex "py print(gdb.parse_and_eval('z')['i'])"  a.out
GNU gdb (GDB) Fedora 10.2-3.fc34
Copyright (C) 2021 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from a.out...
Temporary breakpoint 1 at 0x40110a: file ambig.cc, line 15.
Starting program: /tmp/a.out 

Temporary breakpoint 1, main () at ambig.cc:15
15        Z z;
16        return 0;
89 'Y'

But GDB 12 gives an error:

$ ~/gcc/gdb/11/bin/gdb -ex start -ex n -ex "py
print(gdb.parse_and_eval('z')['i'])"  a.out
GNU gdb (GDB) 12.0.50.20211020-git
Copyright (C) 2021 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-pc-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from a.out...
Temporary breakpoint 1 at 0x40110a: file ambig.cc, line 15.
Starting program: /tmp/a.out 

Temporary breakpoint 1, main () at ambig.cc:15
15        Z z;
16        return 0;
Traceback (most recent call last):
  File "<string>", line 1, in <module>
gdb.error: Request for member 'i' is ambiguous in type 'Z'. Candidates are:
  'int X::i' (Z -> X)
  'char Y::i' (Z -> Y)
Error while executing Python code.

I wonder why there wasn't an error for tuple_member['_M_head_impl'] in the
libstdc++ printer.

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