mlpack fails to build with gcc 10:

[  2%] Building CXX precompiled header src/mlpack/cotire/mlpack_CXX_prefix.hxx.gch
cd /builddir/build/BUILD/mlpack-3.2.2/src/mlpack && /usr/bin/cmake -DCOTIRE_BUILD_TYPE:STRING= -DCOTIRE_VERBOSE:BOOL=1 -P /builddir/build/BUILD/mlpack-3.2.2/CMake/cotire.cmake precompile /builddir/build/BUILD/mlpack-3.2.2/src/mlpack/mlpack_CXX_cotire.cmake /builddir/build/BUILD/mlpack-3.2.2/src/mlpack/cotire/mlpack_CXX_prefix.hxx /builddir/build/BUILD/mlpack-3.2.2/src/mlpack/cotire/mlpack_CXX_prefix.hxx.gch /builddir/build/BUILD/mlpack-3.2.2/src/mlpack/bindings/cli/print_help.cpp
-- execute_process: /usr/bin/c++;-Dmlpack_EXPORTS;-DARMA_NO_DEBUG;-DNDEBUG;-DHAS_STB;-DBOOST_TEST_DYN_LINK;-DHAS_OPENMP;-O2;-g;-pipe;-Wall;-Werror=format-security;-Wp,-D_FORTIFY_SOURCE=2;-Wp,-D_GLIBCXX_ASSERTIONS;-fexceptions;-fstack-protector-strong;-grecord-gcc-switches;-specs=/usr/lib/rpm/redhat/redhat-hardened-cc1;-specs=/usr/lib/rpm/redhat/redhat-annobin-cc1;-m64;-mtune=generic;-fasynchronous-unwind-tables;-fstack-clash-protection;-fcf-protection;-Wall;-Wextra;-ftemplate-depth=1000;-Wno-unused-function;-O3;-fopenmp;-std=gnu++11;-fPIC;-I/builddir/build/BUILD/mlpack-3.2.2/src;-I/builddir/build/BUILD/mlpack-3.2.2/stb;-I/builddir/build/BUILD/mlpack-3.2.2/src/mlpack/..;-x;c++-header;-c;/builddir/build/BUILD/mlpack-3.2.2/src/mlpack/cotire/mlpack_CXX_prefix.hxx;-o;/builddir/build/BUILD/mlpack-3.2.2/src/mlpack/cotire/mlpack_CXX_prefix.hxx.gch
*** WARNING *** there are active plugins, do not report this as a bug unless you can reproduce it without enabling any plugins.
Event                            | Plugins
PLUGIN_FINISH_UNIT               | annobin: Generate final annotations
PLUGIN_START_UNIT                | annobin: Generate global annotations
PLUGIN_ALL_PASSES_START          | annobin: Generate per-function annotations
PLUGIN_ALL_PASSES_END            | annobin: Register per-function end symbol
In file included from /usr/include/armadillo:680,
                 from /builddir/build/BUILD/mlpack-3.2.2/src/mlpack/core/arma_extend/arma_extend.hpp:44,
                 from /builddir/build/BUILD/mlpack-3.2.2/src/mlpack/prereqs.hpp:108,
                 from /builddir/build/BUILD/mlpack-3.2.2/src/mlpack/cotire/mlpack_CXX_prefix.cxx:4,
                 from /builddir/build/BUILD/mlpack-3.2.2/src/mlpack/cotire/mlpack_CXX_prefix.hxx:4:
/usr/include/armadillo_bits/op_find_meat.hpp: In static member function 'static void arma::op_find::apply(arma::Mat<long long unsigned int>&, const arma::mtOp<long long unsigned int, T1, arma::op_find>&)':
/usr/include/armadillo_bits/op_find_meat.hpp:433:89: internal compiler error: in nothrow_spec_p, at cp/except.c:1247
  433 |       out = (k > 0 && k <= n_nz) ? indices.rows(0,      k-1   ) : indices.rows(0, n_nz-1);
      |             

This failure happened during our Python 3.9 rebuilds. While not related to Python 3.9, it still blocks it.

For the build logs, see:
https://copr-be.cloud.fedoraproject.org/results/@python/python3.9/fedora-rawhide-x86_64/01173459-mlpack/

For all our attempts to build mlpack with Python 3.9, see:
https://copr.fedorainfracloud.org/coprs/g/python/python3.9/package/mlpack/