[Bug libstdc++/55043] [4.7/4.8 Regression] issue with nesting unordered_map containing unique_ptr into vector

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55043

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |daniel.kruegler at
                   |                            |googlemail dot com

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-10-24 01:24:02 UTC ---
The problem reduces to this:

  #include <type_traits>
  #include <utility>

  struct M {
    M(M&&) { }
  };

  template<typename T>
    struct U
    {
      U(const U& n) : m(n.m) { }
      U(U&& n);
      T m;
    };

  static_assert( std::is_copy_constructible<U<M>>::value, "huh?" );

We need that static_assertion to fail because the type is *not* copy
constructible.

But it passes, because this compiles OK:

    decltype(::new U<M>(std::declval<const U<M>&>())) n;


Even though the definition of U(const U&) is ill-formed for a non-copyable
template argument, using that constructor as an unevaluated operand compiles
OK.

Daniel, is this a bug in is_constructible?