This issue appear during freeorion compilation. Test case compile fine with GCC 7 and Clang 5 but fail with GCC 8.

$ cat StyleFactory.cpp
namespace std {
template <typename> class shared_ptr {};
class a;
template <class> class b {
  a *g() const;
  shared_ptr<a> e;
};
template <class f> a *b<f>::g() const { return e; }
}

$ g++ --version
g++ (GCC) 8.0.1 20180123 (experimental)
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ g++ -O2 -fPIC -std=c++11 -o StyleFactory.cpp.i.o -c StyleFactory.cpp                                             StyleFactory.cpp: In member function ‘std::a* std::b< <template-parameter-1-1> >::g() const’:
StyleFactory.cpp:8:48: error: cannot convert ‘const std::shared_ptr<std::a>’ to ‘std::a*’ in return
 template <class f> a *b<f>::g() const { return e; }
                                                ^

GCC 7 works fine.

$ g++ --version
g++ (GCC) 7.2.1 20180123
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ g++ -O2 -fPIC -std=c++11 -o StyleFactory.cpp.i.o -c StyleFactory.cpp
$ echo $?
0

Clang 5 also work fine.

$ clang++ --version
clang version 5.0.1
Target: x86_64-unknown-linux-gnu
Thread model: posix

$ clang++ -O2 -fPIC -std=c++11 -o StyleFactory.cpp.i.o -c StyleFactory.cpp
$ echo $?
0