I fail to see how an explicit ParseHex template instantiation fails to also instantiate TryParseHex.
Nonetheless, to work around a compiler bug, change the explicit instantiation from ParseHex to TryParseHex. (ParseHex is inline anyway and will be instantiated by the compiler either way).
Fixes #25227 (comment) :
0 CXXLD bitcoind
1/usr/lib/gcc-cross/powerpc64le-linux-gnu/11/../../../../powerpc64le-linux-gnu/bin/ld: libbitcoin_node.a(libbitcoin_node_a-net_processing.o): in function `(anonymous namespace)::PeerManagerImpl::ProcessMessage(CNode&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, CDataStream&, std::chrono::duration<long, std::ratio<1l, 1000000l> >, std::atomic<bool> const&)':
2net_processing.cpp:(.text+0x29660): undefined reference to `std::optional<std::vector<unsigned char, std::allocator<unsigned char> > > TryParseHex<unsigned char>(std::basic_string_view<char, std::char_traits<char> >)'
3/usr/lib/gcc-cross/powerpc64le-linux-gnu/11/../../../../powerpc64le-linux-gnu/bin/ld: libbitcoin_node.a(libbitcoin_node_a-rest.o): in function `rest_getutxos(std::any const&, HTTPRequest*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)':
4rest.cpp:(.text+0x83b4): undefined reference to `std::optional<std::vector<unsigned char, std::allocator<unsigned char> > > TryParseHex<unsigned char>(std::basic_string_view<char, std::char_traits<char> >)'
5/usr/lib/gcc-cross/powerpc64le-linux-gnu/11/../../../../powerpc64le-linux-gnu/bin/ld: libbitcoin_node.a(libbitcoin_node_a-torcontrol.o): in function `std::vector<unsigned char, std::allocator<unsigned char> > ParseHex<unsigned char>(std::basic_string_view<char, std::char_traits<char> >)':
6torcontrol.cpp:(.text._Z8ParseHexIhESt6vectorIT_SaIS1_EESt17basic_string_viewIcSt11char_traitsIcEE[_Z8ParseHexIhESt6vectorIT_SaIS1_EESt17basic_string_viewIcSt11char_traitsIcEE]+0x2c): undefined reference to `std::optional<std::vector<unsigned char, std::allocator<unsigned char> > > TryParseHex<unsigned char>(std::basic_string_view<char, std::char_traits<char> >)'
7/usr/lib/gcc-cross/powerpc64le-linux-gnu/11/../../../../powerpc64le-linux-gnu/bin/ld: libbitcoin_common.a(libbitcoin_common_a-external_signer.o): in function `ExternalSigner::SignTransaction(PartiallySignedTransaction&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&)':
8external_signer.cpp:(.text+0x8d84): undefined reference to `std::optional<std::vector<unsigned char, std::allocator<unsigned char> > > TryParseHex<unsigned char>(std::basic_string_view<char, std::char_traits<char> >)'
9collect2: error: ld returned 1 exit status
