[Bug c++/86429] New: lambda capture breaks constexpr-ness

[zub at linux dot fjfi.cvut.cz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86429

            Bug ID: 86429
           Summary: lambda capture breaks constexpr-ness
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zub at linux dot fjfi.cvut.cz
  Target Milestone: ---

This issue seems similar to bug 82643 but unlike that bug, this code compiles
fine in g++ 7.3 and fails to compile in g++ 8.1. Trying it out at wandabox.org
with gcc HEAD (201807) also results in compilation error.

The code requires boost hana:

====
#include <iostream>
#include <boost/hana.hpp>

int main()
{
        namespace hana = boost::hana;

        constexpr auto map = hana::make_map(hana::make_pair(hana::type_c<int>,
"foo"));
        [&](auto v) {
                constexpr auto x = boost::hana::find(map, v);
                std::cout << *x << '\n';
        }(hana::type_c<int>);

        return 0;
}
====

With g++ 7.3 and clang++ it compiles OK and when run, prints out "foo\n" on
stdout. With g++ 8.1 I get:

$ g++-8 -std=c++17 gcc-8.1-not_contant.cc 
gcc-8.1-not_contant.cc: In instantiation of ‘main()::<lambda(auto:1)> [with
auto:1 = boost::hana::type_impl<int>::_]’:
gcc-8.1-not_contant.cc:12:21:   required from here
gcc-8.1-not_contant.cc:10:18: error: ‘__closure’ is not a constant expression
   constexpr auto x = boost::hana::find(map, v);
                  ^

While I'm not 100% sure the code is correct, even the error message is strange
(__closure?).