The Standard Library has historically been at least a tiny bit careful (though not as careful as I'd prefer) to avoid adding "greedy" templates that collide with plausible user names.

See https://quuxplusone.github.io/blog/2018/11/04/std-hash-value/ for more details.

However, in the specific case of functions taking only enum types, I think you're in the clear — because the associated namespaces for nested enum types do not include the kitchen sink! For example, A<B>::E is associated only with A, never with B or even with the base classes of A.

Here's my most valiant effort to make ADL break your proposal... and my effort failed: https://godbolt.org/z/8_Rix-

–Arthur