[Bug c++/86126] New: Note when linking libraries built with different -std options

[gburca-gnu at ebixio dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86126

            Bug ID: 86126
           Summary: Note when linking libraries built with different -std
                    options
           Product: gcc
           Version: 8.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gburca-gnu at ebixio dot com
  Target Milestone: ---

Created attachment 44265
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44265&action=edit
Bug demo

Linking together libraries that have been built with LTO and different -std
options causes g++ to issue notes of the form: type 'A' should match type 'A'.

Specifically, with code below, the output is:

..../gcc/8.1.0-1/include/c++/8.1.0/sstream:80:57: note: type ‘struct
__string_type’ should match type ‘struct __string_type’
       typedef basic_string<char_type, _Traits, _Alloc>  __string_type;
                                                         ^
..../gcc/8.1.0-1/include/c++/8.1.0/sstream:540:57: note: type ‘struct
__stringbuf_type’ should match type ‘struct __stringbuf_type’
       typedef basic_stringbuf<_CharT, _Traits, _Alloc>  __stringbuf_type;
                                                         ^

I've tested with gcc 7.2.0 and it also issues a note about 'struct
__string_type'

Based on what I see at https://gcc.gnu.org/onlinedocs/libstdc++/manual/abi.html
and
https://stackoverflow.com/questions/46746878/is-it-safe-to-link-c17-c14-and-c11-objects/49119902#49119902
it should be possible to link together objects built with the same compiler but
different -std options.

The libraries are built with:
  g++ -fPIC -c liba.cpp -std=gnu++14 -o a.o ${CFLAGS}
  g++ -fPIC -c libb.cpp -std=gnu++17 -o b.o ${CFLAGS}
  ar cru liba.a a.o
  ar cru libb.a b.o

They are linked together with:
  g++ main.cpp -std=gnu++17 -L. -la -lb -o main ${CFLAGS}
-Wl,-rpath,${CXX_PATH}/lib64

And:
  CFLAGS := -flto -ffat-lto-objects