[Bug c++/86556] New: <random>, most vexing parse, and diagnostic output

["nightstrike at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86556

            Bug ID: 86556
           Summary: <random>, most vexing parse, and diagnostic output
           Product: gcc
           Version: 8.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nightstrike at gmail dot com
  Target Milestone: ---

This works:

#include <random>
bool f() {
        std::random_device r;
        std::mt19937 gen(r());
        std::bernoulli_distribution d(0.5);
        return d(gen);
}


This easy to make but clearly wrong typo produces incomprehensible (to me)
errors:

#include <random>
bool f() {
        std::mt19937 gen(std::random_device());
        std::bernoulli_distribution d(0.5);
        return d(gen);
}


In file included from /gcc/gcc8/include/c++/8.1.0/random:51,
                 from rand.cc:1:
/gcc/gcc8/include/c++/8.1.0/bits/random.tcc: In instantiation of '_RealType
std::generate_canonical(_UniformRandomNumberGenerator&) [with _RealType =
double; long unsigned int __bits = 53; _UniformRandomNumberGenerator =
std::mersenne_twister_engine<long unsigned int, 32, 624, 397, 31, 2567483615,
11, 4294967295, 7, 2636928640, 15, 4022730752, 18,
1812433253>(std::random_device (*)())]':
/gcc/gcc8/include/c++/8.1.0/bits/random.h:181:38:   required from '_DInputType
std::__detail::_Adaptor<_Engine, _DInputType>::operator()() [with _Engine =
std::mersenne_twister_engine<long unsigned int, 32, 624, 397, 31, 2567483615,
11, 4294967295, 7, 2636928640, 15, 4022730752, 18,
1812433253>(std::random_device (*)()); _DInputType = double]'
/gcc/gcc8/include/c++/8.1.0/bits/random.h:3562:14:   required from
'std::bernoulli_distribution::result_type
std::bernoulli_distribution::operator()(_UniformRandomNumberGenerator&, const
std::bernoulli_distribution::param_type&) [with _UniformRandomNumberGenerator =
std::mersenne_twister_engine<long unsigned int, 32, 624, 397, 31, 2567483615,
11, 4294967295, 7, 2636928640, 15, 4022730752, 18,
1812433253>(std::random_device (*)()); std::bernoulli_distribution::result_type
= bool]'
/gcc/gcc8/include/c++/8.1.0/bits/random.h:3553:49:   required from
'std::bernoulli_distribution::result_type
std::bernoulli_distribution::operator()(_UniformRandomNumberGenerator&) [with
_UniformRandomNumberGenerator = std::mersenne_twister_engine<long unsigned int,
32, 624, 397, 31, 2567483615, 11, 4294967295, 7, 2636928640, 15, 4022730752,
18, 1812433253>(std::random_device (*)());
std::bernoulli_distribution::result_type = bool]'
rand.cc:5:14:   required from here
/gcc/gcc8/include/c++/8.1.0/bits/random.tcc:3323:63: error: request for member
'max' in '__urng', which is of non-class type
'std::mersenne_twister_engine<long unsigned int, 32, 624, 397, 31, 2567483615,
11, 4294967295, 7, 2636928640, 15, 4022730752, 18,
1812433253>(std::random_device (*)())'
       const long double __r = static_cast<long double>(__urng.max())
                                                        ~~~~~~~^~~
/gcc/gcc8/include/c++/8.1.0/bits/random.tcc:3324:42: error: request for member
'min' in '__urng', which is of non-class type
'std::mersenne_twister_engine<long unsigned int, 32, 624, 397, 31, 2567483615,
11, 4294967295, 7, 2636928640, 15, 4022730752, 18,
1812433253>(std::random_device (*)())'
        - static_cast<long double>(__urng.min()) + 1.0L;
                                   ~~~~~~~^~~
/gcc/gcc8/include/c++/8.1.0/bits/random.tcc:3333:29: error: too few arguments
to function
    __sum += _RealType(__urng() - __urng.min()) * __tmp;
                       ~~~~~~^~
/gcc/gcc8/include/c++/8.1.0/bits/random.tcc:3333:41: error: request for member
'min' in '__urng', which is of non-class type
'std::mersenne_twister_engine<long unsigned int, 32, 624, 397, 31, 2567483615,
11, 4294967295, 7, 2636928640, 15, 4022730752, 18,
1812433253>(std::random_device (*)())'
    __sum += _RealType(__urng() - __urng.min()) * __tmp;
                                  ~~~~~~~^~~