In order to provide the amortized constant time complexity required by range, filter_view::begin caches the result in *this. This modifies the internal state of *this and thus cannot be done in a const member function.
This is a known bug in Clang’s concepts implementation, see https://github.com/llvm/llvm-project/issues/44178 The change https://reviews.llvm.org/D119544 is supposed to fix it, but it hasn’t been committed to Clang trunk yet (as of June 22 2022).
After looking into defining a view interface, I decided to use range-v3. There are several things missing from std::ranges https://stackoverflow.com/a/68172842/11998382, and until they are added in future standards, it is sensible to use range-v3 rather than repeatedly attempting non-trivial implementations yourself.