static_assert dependent on non-type template parameter (different behavior on gcc and clang)

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Both compilers are correct. From [temp.res]/8:

If no valid specialization can be generated for a template, and that template is not instantiated, the template is ill-formed, no diagnostic
required.

There does not exist a valid specialization that can be generated from the primary template Hitchhiker, so it is ill-formed, no diagnostic required. clang chooses to issue a diagnostic anyway.

If you only want to allow 42, then simply don’t define the general template:

template <int > struct Hitchhiker;
template <> struct Hitchhiker<42> {};

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