In both [temp.arg.template]-2 and [temp.spec.partial.general]-1, the standard requires that in each translation unit the partial specializations are reachable in all points of implicit or explicit instantiations where they would have been selected if they had been reachable.That is, it is not allowed to declare specializations after instantiations where the former would have been selected.
However, I guess the text should consider a more general case when the early declaration would lead to an ambiguity, that is, the postponed declaration would be in the set of best selectable specializations, not necessarily the unique winner. Technically, speaking, to refer that no more specialized specializations should be before the instantiation (really, it does not matter before or after as, in case of after, the error would exist for the more specialized one)].
e.g.
template<int N>
class C{};
template<int N>
requires (N < 6)
class C<N>{};
C<4> x; // would have matched both constrained specializations, that are not partially ordered when compared each other
template<int N>
requires (N < 5)
class C<N>{};
Indeed, gcc 14.2 raises a compilation error when second specialization is declared ('declaration of 'class CL<N>' ambiguates earlier template instantiation for 'class CL<4>'').
Both mvsc 19.4 and clang 19.1 do not.
However, the standard does not require diagnostic.
Mauro.