Umm.  Cryptic, yes

In file included from hb-face.cc:35:
hb-ot-cmap-table.hh: In member function 'void OT::CmapSubtableFormat4::_compiles_assertion_on_line_388() const':
hb-ot-cmap-table.hh:388: error: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:
hb-open-type.hh:354: note: candidate 1: const Type& OT::UnsizedArrayOf<Type>::operator[](unsigned int) const [with Type = OT::IntType<short unsigned int, 2u>]
hb-ot-cmap-table.hh:388: note: candidate 2: operator[](const T*, int) <built-in>
hb-ot-cmap-table.hh: In member function 'void OT::CmapSubtableFormat4::_instance_assertion_on_line_388() const':
hb-ot-cmap-table.hh:388: error: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:
hb-open-type.hh:354: note: candidate 1: const Type& OT::UnsizedArrayOf<Type>::operator[](unsigned int) const [with Type = OT::IntType<short unsigned int, 2u>]
hb-ot-cmap-table.hh:388: note: candidate 2: operator[](const T*, int) <built-in>
hb-face.cc: In function 'hb_blob_t* _hb_face_builder_data_reference_blob(hb_face_builder_data_t*)':
hb-face.cc:650: error: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:
hb-vector.hh:81: note: candidate 1: Type& hb_vector_t<Type, PreallocedCount>::operator[](unsigned int) [with Type = hb_face_builder_data_t::table_entry_t, unsigned int PreallocedCount = 32u]
hb-face.cc:650: note: candidate 2: operator[](T*, int) <built-in>
hb-face.cc:650: error: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:
hb-vector.hh:81: note: candidate 1: Type& hb_vector_t<Type, PreallocedCount>::operator[](unsigned int) [with Type = hb_face_builder_data_t::table_entry_t, unsigned int PreallocedCount = 32u]
hb-face.cc:650: note: candidate 2: operator[](const T*, int) <built-in>
hb-face.cc:651: error: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:
hb-vector.hh:81: note: candidate 1: Type& hb_vector_t<Type, PreallocedCount>::operator[](unsigned int) [with Type = hb_face_builder_data_t::table_entry_t, unsigned int PreallocedCount = 32u]
hb-face.cc:651: note: candidate 2: operator[](T*, int) <built-in>
hb-face.cc:651: error: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:
hb-vector.hh:81: note: candidate 1: Type& hb_vector_t<Type, PreallocedCount>::operator[](unsigned int) [with Type = hb_face_builder_data_t::table_entry_t, unsigned int PreallocedCount = 32u]
hb-face.cc:651: note: candidate 2: operator[](const T*, int) <built-in>
2 files changed