Template with explicit instantiation causes constructor precondition to fail

[PeterBindels-TomTom]

template <size_t bits>
class t {
  explicit t(std::span<const uint8_t> f) pre(f.size() == bits/8);
};

template <>
t<128>::t(std::span<const uint8_t> f) {}

Does not want to reduce much further than this. It turns out that the explicit instantiation is required for this to trigger, but I have no idea why.

[EricWF]

Does not want to reduce much further than this. It turns out that the explicit instantiation is required for this to trigger, but I have no idea why.

I think that's an explicit specialization, rather than instantiation.

The explicit instantiation "works", but also this test case shook out like 3 more bugs:

https://godbolt.org/z/dEf75dG9d