For working draft on 30th-Oct-2024 (https://eel.is/c++draft/)

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.