[ The paragraph in question was removed by P2448R2, adopted at the July, 2022 meeting. ]

According to 9.2.6 [dcl.constexpr] paragraph 6,

If no specialization of the template would satisfy the requirements for a constexpr function or constexpr constructor when considered as a non-template function or constructor, the template is ill-formed; no diagnostic required.

This should say “instantiated template specialization” instead of just “specialization” to clarify that an explicit specialization is not in view here.