Disable -Werror=implicit-fallthrough on GCC7 and later.  Taken from upstream:
https://github.com/Tencent/rapidjson/commit/fe19b7b6016d446722621fb407738209d1a911e8
https://github.com/Tencent/rapidjson/commit/cba45fe9de6923b858edb0780e257b7257aa4f7b

diff --git a/include/rapidjson/internal/regex.h b/include/rapidjson/internal/regex.h
index 422a5240..d4039716 100644
--- a/include/rapidjson/internal/regex.h
+++ b/include/rapidjson/internal/regex.h
@@ -29,6 +29,9 @@ RAPIDJSON_DIAG_OFF(implicit-fallthrough)
 #ifdef __GNUC__
 RAPIDJSON_DIAG_PUSH
 RAPIDJSON_DIAG_OFF(effc++)
+#if __GNUC__ >= 7
+RAPIDJSON_DIAG_OFF(implicit-fallthrough)
+#endif
 #endif
 
 #ifdef _MSC_VER

Cast destination pointers to avoid a GCC error about memmove writing to
a different type.

Adapted from this upstream commit:
https://github.com/Tencent/rapidjson/commit/fa5963a2f5b231ee2babff771f169ccca22870ed

diff --git a/include/rapidjson/document.h b/include/rapidjson/document.h
--- a/include/rapidjson/document.h
+++ b/include/rapidjson/document.h
@@ -1425,7 +1425,7 @@ public:
         MemberIterator pos = MemberBegin() + (first - MemberBegin());
         for (MemberIterator itr = pos; itr != last; ++itr)
             itr->~Member();
-        std::memmove(&*pos, &*last, static_cast<size_t>(MemberEnd() - last) * sizeof(Member));
+        std::memmove(static_cast<void*>(&*pos), &*last, static_cast<size_t>(MemberEnd() - last) * sizeof(Member));
         data_.o.size -= static_cast<SizeType>(last - first);
         return pos;
     }
@@ -1628,8 +1628,8 @@ public:
         RAPIDJSON_ASSERT(last <= End());
         ValueIterator pos = Begin() + (first - Begin());
         for (ValueIterator itr = pos; itr != last; ++itr)
-            itr->~GenericValue();       
-        std::memmove(pos, last, static_cast<size_t>(End() - last) * sizeof(GenericValue));
+            itr->~GenericValue();
+        std::memmove(static_cast<void*>(pos), last, static_cast<size_t>(End() - last) * sizeof(GenericValue));
         data_.a.size -= static_cast<SizeType>(last - first);
         return pos;
     }
@@ -1936,7 +1936,7 @@ private:
         if (count) {
             GenericValue* e = static_cast<GenericValue*>(allocator.Malloc(count * sizeof(GenericValue)));
             SetElementsPointer(e);
-            std::memcpy(e, values, count * sizeof(GenericValue));
+            std::memcpy(static_cast<void*>(e), values, count * sizeof(GenericValue));
         }
         else
             SetElementsPointer(0);
@@ -1949,7 +1949,7 @@ private:
         if (count) {
             Member* m = static_cast<Member*>(allocator.Malloc(count * sizeof(Member)));
             SetMembersPointer(m);
-            std::memcpy(m, members, count * sizeof(Member));
+            std::memcpy(static_cast<void*>(m), members, count * sizeof(Member));
         }
         else
             SetMembersPointer(0);