[Bug c++/101500] [C++17] [DR2311] gcc accepts the code with extra curly braces

["jason at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101500

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[9/10/11/12 Regression] gcc |[C++17] [DR2311] gcc
                   |accepts the code with extra |accepts the code with extra
                   |curly braces                |curly braces
             Status|NEW                         |SUSPENDED
                URL|                            |wg21.link/cwg2311

--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> ---
This seems like an instance of issue 2311: with C++17 mandatory copy elision,
do we really want an actual call to the move constructor in this case?  The
testcase is rejected in C++14 mode, as expected.

The standard says that we try to call a constructor, initializing the argument
with {A{}}.  So we in turn look for an A constructor to initialize the
argument, initializing its argument from A{}.  Under 2311, does this result in
still an rvalue?  And the enclosing constructor call, still the same rvalue?

This seems parallel to

const A a = A(A(A()));

which is also accepted by clang in C++17 mode.

Perhaps we want to raise the priority of issue 2311.