Project does not build with clang

[per-gron]

When building CBAG with clang 9.0.1-12 I get the following error

In file included from external/com_bcanalog_cbag/src/cbag/gdsii/write.cpp:52:
In file included from external/com_bcanalog_cbag/include/cbag/gdsii/write.h:63:
In file included from external/com_bcanalog_cbag/include/cbag/layout/cellview_fwd.h:56:
In file included from external/com_bcanalog_cbag/include/cbag/layout/instance.h:56:
In file included from external/com_bcanalog_cbag/include/cbag/common/param_map.h:51:
external/com_bcanalog_cbag/include/cbag/util/sorted_map.h:85:9: error: no member 'key_compare' in 'cbag::util::sorted_map<std::__cxx11::basic_string<char>, std::variant<long, double, bool, std::__cxx11::basic_string<char>, cbag::time_struct, cbag::binary_t>, std::less<std::__cxx11::basic_string<char> > >'; it has not yet been instantiated
        key_compare key_comp_;
        ^
external/com_bcanalog_cbag/include/cbag/util/sorted_vector.h:77:13: note: in instantiation of member class 'cbag::util::sorted_map<std::__cxx11::basic_string<char>, std::variant<long, double, bool, std::__cxx11::basic_string<char>, cbag::time_struct, cbag::binary_t>, std::less<std::__cxx11::basic_string<char> > >::value_compare' requested here
    Compare comp_;
            ^
external/com_bcanalog_cbag/include/cbag/util/sorted_map.h:64:32: note: in instantiation of template class 'cbag::util::sorted_vector<std::pair<std::__cxx11::basic_string<char>, std::variant<long, double, bool, std::__cxx11::basic_string<char>, cbag::time_struct, cbag::binary_t> >, cbag::util::sorted_map<std::__cxx11::basic_string<char>, std::variant<long, double, bool, std::__cxx11::basic_string<char>, cbag::time_struct, cbag::binary_t>, std::less<std::__cxx11::basic_string<char> > >::value_compare>' requested here
    using size_type = typename vector_type::size_type;
                               ^
external/com_bcanalog_cbag/include/cbag/layout/instance.h:70:15: note: in instantiation of template class 'cbag::util::sorted_map<std::__cxx11::basic_string<char>, std::variant<long, double, bool, std::__cxx11::basic_string<char>, cbag::time_struct, cbag::binary_t>, std::less<std::__cxx11::basic_string<char> > >' requested here
    param_map params;
              ^
external/com_bcanalog_cbag/include/cbag/util/sorted_map.h:66:11: note: not-yet-instantiated member is declared here
    using key_compare = Compare;
          ^
1 error generated.

The following patch fixes it

diff --git include/cbag/util/sorted_map.h include/cbag/util/sorted_map.h
index 8d41ae6..6b8ef4d 100644
--- include/cbag/util/sorted_map.h
+++ include/cbag/util/sorted_map.h
@@ -60,10 +60,10 @@ template <class Key, class T, class Compare = std::less<Key>> class sorted_map {
     using key_type = Key;
     using mapped_type = T;
     using value_type = std::pair<key_type, mapped_type>;
+    using key_compare = Compare;
     using vector_type = sorted_vector<value_type, value_compare>;
     using size_type = typename vector_type::size_type;
     using difference_type = typename vector_type::difference_type;
-    using key_compare = Compare;
     using reference = value_type &;
     using const_reference = const value_type &;
     using iterator = typename vector_type::iterator;