It seems like Mojo uses unordered_map for map types in its generated C++ files. This can cause problems if the key is a type that does not have a hash function associated with it.

The change below demonstrates and example of this problem:
https://chromium-review.googlesource.com/c/chromium/src/+/951892

The workaround to this problem was to create a typemap:
https://chromium-review.googlesource.com/c/chromium/src/+/942307