https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84075
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> --- Created attachment 44642 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44642&action=edit creduce output Here's the result of many hours of creduction with this interestingness test: #!/bin/bash ~/src/llvm/latest/bin/clang++ -std=c++17 petri.cc -c -w || exit 1 LANG=C g++ -std=c++17 petri.cc -c -w 2>out && exit 1 awk "/ error: invalid application of 'sizeof' to incomplete type 'boost::serialization::U'/ { found=1 ; --errors } / error: / { ++errors } END { exit found ? errors : 99 }" out It compiles OK with clang but with g++ -std=c++17 -w the error is: petri-net.cc: In instantiation of 'struct a::e<boost::serialization::U>': petri-net.cc:44:50: required from 'class a::k<a::g<boost::serialization::U, 0, int> >' petri-net.cc:49:47: required from 'class a::g<boost::serialization::U, 0, int>' petri-net.cc:103:7: required from 'static void boost::serialization::bh<au, d>::bi(au) [with au = boost::cl; d = a::aa<a::g<int>, a::g<a::g<int>, 0, int> >]' petri-net.cc:109:3: required from 'void boost::serialization::bk(au, d, int) [with au = boost::cl; d = a::aa<a::g<int>, a::g<a::g<int>, 0, int> >]' petri-net.cc:148:5: [ skipping 32 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ] petri-net.cc:177:8: required from 'static void boost::cj<au>::ck::bi(au, d) [with d = a::g<int>; au = boost::cl]' petri-net.cc:182:11: required from 'static void boost::cj<au>::bi(au, d) [with d = a::g<int>; au = boost::cl]' petri-net.cc:188:9: required from 'void boost::bj(au, d) [with au = boost::cl; d = a::g<int>]' petri-net.cc:201:7: required from 'void boost::co::cn(d) [with d = a::g<int>]' petri-net.cc:208:5: required from 'void boost::cp::cn(boost::serialization::bd<d>) [with d = a::g<int>]' petri-net.cc:195:46: required from 'boost::cl& boost::interface_iarchive< <template-parameter-1-1> >::operator>>(d) [with d = boost::serialization::bd<a::g<int> >; <template-parameter-1-1> = int]' petri-net.cc:225:40: required from here petri-net.cc:37:15: error: invalid application of 'sizeof' to incomplete type 'boost::serialization::U' 37 | typename b< sizeof(d) >::type NonInteger; | ^~~~~~~~~ Bizarrely, it compiles OK if __attribute__((__used__)) is removed on line 167.
