[Bug libstdc++/87855] std::optional<T> only copy-constructible if T is trivially copy-constructible

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87855

--- Comment #19 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Author: redi
Date: Tue Jan  8 23:00:46 2019
New Revision: 267742

URL: https://gcc.gnu.org/viewcvs?rev=267742&root=gcc&view=rev
Log:
PR libstdc++/87855 fix optional for types with non-trivial copy/move

When the contained value is not trivially copy (or move) constructible
the union's copy (or move) constructor will be deleted, and so the
_Optional_payload delegating constructors are invalid. G++ fails to
diagnose this because it incorrectly performs copy elision in the
delegating constructors. Clang does diagnose it (llvm.org/PR40245).

The solution is to avoid performing any copy (or move) when the
contained value's copy (or move) constructor isn't trivial. Instead the
contained value can be constructed by calling _M_construct. This is OK,
because the relevant constructor doesn't need to be constexpr when the
contained value isn't trivially copy (or move) constructible.

Additionally, this patch removes a lot of code duplication in the
_Optional_payload partial specializations and the _Optional_base partial
specialization, by hoisting it into common base classes.

The Python pretty printer for std::optional needs to be adjusted to
support the new layout. Retain support for the old layout, and add a
test to verify that the support still works.

        PR libstdc++/87855
        * include/std/optional (_Optional_payload_base): New class template
        for common code hoisted from _Optional_payload specializations. Use
        a template for the union, to allow a partial specialization for
        types with non-trivial destructors. Add constructors for in-place
        initialization to the union.
        (_Optional_payload(bool, const _Optional_payload&)): Use _M_construct
        to perform non-trivial copy construction, instead of relying on
        non-standard copy elision in a delegating constructor.
        (_Optional_payload(bool, _Optional_payload&&)): Likewise for
        non-trivial move construction.
        (_Optional_payload): Derive from _Optional_payload_base and use it
        for everything except the non-trivial assignment operators, which are
        defined as needed.
        (_Optional_payload<false, C, M>): Derive from the specialization
        _Optional_payload<true, false, false> and add a destructor.
        (_Optional_base_impl::_M_destruct, _Optional_base_impl::_M_reset):
        Forward to corresponding members of _Optional_payload.
        (_Optional_base_impl::_M_is_engaged, _Optional_base_impl::_M_get):
        Hoist common members from _Optional_base.
        (_Optional_base): Make all members and base class public.
        (_Optional_base::_M_get, _Optional_base::_M_is_engaged): Move to
        _Optional_base_impl.
        * python/libstdcxx/v6/printers.py (StdExpOptionalPrinter): Add
        support for new std::optional layout.
        * testsuite/libstdc++-prettyprinters/compat.cc: New test.

Added:
    trunk/libstdc++-v3/testsuite/libstdc++-prettyprinters/compat.cc
Modified:
    trunk/libstdc++-v3/ChangeLog
    trunk/libstdc++-v3/include/std/optional
    trunk/libstdc++-v3/python/libstdcxx/v6/printers.py