[Bug libstdc++/37986] std::tr1::variate_generator does not conform to TR1.

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=37986

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Manuel Holtgrewe from comment #0)
>   std::tr1::variate_generator<
>     std::tr1::mt19937&,
>     std::tr1::uniform_real<double>
>     > g(mt, dist);

This case is only fixed for -std=gnu++98 mode. With -std=c++98 there is no
reference collapsing and the std::tr1::__detail::_Adaptor class template tries
to form a reference to a reference.

From a look at the code, it appears that our variate_generator is not
conforming, because its engine_value_type is the adaptor class, not the Engine
argument.

I don't intend to fix this, I'm just noting it here for posterity.