[Bug libstdc++/71364] [7 regression] recent tuple changes break range-v3 merge.cpp

["Casey at Carter dot net" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71364

Casey Carter <Casey at Carter dot net> changed:

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                 CC|                            |Casey at Carter dot net

--- Comment #4 from Casey Carter <Casey at Carter dot net> ---
The bug is in range-v3 and the Ranges TS
(https://github.com/ericniebler/stl2/issues/172) which was triggered by libc++
tuple changes on April 15 (https://github.com/ericniebler/range-v3/issues/324). 

Our specification of tagged<Base, ...> - the template wrapper that implements
tagged_pair and tagged_tuple - inherits constructors from Base but has no
specific constructors that take Base const& or Base&&. Consequently conversions
from Base have been using tuple/pair's converting constructor instead of the
copy/move constructor. LWG2549
(http://cplusplus.github.io/LWG/lwg-active.html#2549) makes this not work
anymore: the converting constructor refuses to accept argument expressions with
the tuple's same type that should be using the copy/move constructors.

This has been fixed in range-v3, but not yet in the Ranges TS.