[Bug libstdc++/86751] [6/7/8 Regression] Ambiguous operator= overload for std::pair<T1, T2>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86751

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Author: redi
Date: Mon Oct 15 11:52:48 2018
New Revision: 265162

URL: https://gcc.gnu.org/viewcvs?rev=265162&root=gcc&view=rev
Log:
PR libstdc++/86751 default assignment operators for std::pair

The solution for PR 77537 causes ambiguities due to the extra copy
assignment operator taking a __nonesuch_no_braces parameter. By making
the base class non-assignable we don't need the extra deleted overload
in std::pair. The copy assignment operator will be implicitly deleted
(and the move assignment operator not declared) as needed. Without the
additional user-provided operator in std::pair the ambiguity is avoided.

Backport from mainline
2018-07-31  Jonathan Wakely  <jwakely@redhat.com>

        PR libstdc++/86751
        * include/bits/stl_pair.h (__pair_base): New class with deleted copy
        assignment operator.
        (pair): Derive from __pair_base.
        (pair::operator=): Remove deleted overload.
        * python/libstdcxx/v6/printers.py (StdPairPrinter): New pretty printer
        so that new base class isn't shown in GDB.
        * testsuite/20_util/pair/86751.cc: New test.
        * testsuite/20_util/pair/ref_assign.cc: New test.

Added:
    branches/gcc-6-branch/libstdc++-v3/testsuite/20_util/pair/86751.cc
    branches/gcc-6-branch/libstdc++-v3/testsuite/20_util/pair/ref_assign.cc
Modified:
    branches/gcc-6-branch/libstdc++-v3/ChangeLog
    branches/gcc-6-branch/libstdc++-v3/include/bits/stl_pair.h
    branches/gcc-6-branch/libstdc++-v3/python/libstdcxx/v6/printers.py