Using const string as key type instead of plain string fails with libc++,
the standard library implementation used by clang. The libc++ implementation
assumes that map keys can be copied, and the standard requires map keys to be
copyable.
Using const string as a value also fails with clang, I am not 100% sure why.
It does severely restrict how you can interact with the container, and seems to
make it noncopyable. The error is reported from DynamicState<GroupTree>::update(),
the std::fill() call.
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