[Bug c++/66135] New: trailing return type generic lambda

["barry.revzin at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66135

            Bug ID: 66135
           Summary: trailing return type generic lambda
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

Consider this attempt at implementing fmap for optionals in C++:

#include <boost/optional.hpp>
#include <iostream>

template <typename Func>
auto fmap(Func f)
{
    return [=](auto arg) -> boost::optional<decltype(f(*arg))> { // **
        if (arg) {
            return f(*arg); 
        }
        else {
            return boost::none;
        }
    };
}


int main() {
    auto id = fmap([](int i){ return i; });

    auto res = id(boost::optional<int>{42});
    std::cout << *res << std::endl;
}

This code compiles in clang 3.6, but on gcc 5.1 issues an error on the
indicated line:

    error: invalid type argument of unary '*' (have 'auto:1')