Namespace clashes that cause import errors, in my limited experience, have always been in the global namespace, precisely because it is the only namespace that is likely to have the same name as another ("global" is global, and ipso facto, default).
Of course, if the same enum is defined in two separate files, in the same named namespace, then you have the same clash situation. That is only to be expected: a clash is a clash, in any namespace, not just the global one. In which case, the problem is not in declaring the enum in its own namespace; the problem is still in the clash in the namespace.
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