dotproducttest fails with libc++

See build log: https://gitlab.dune-project.org/core/dune-istl/-/jobs/45087

The build log does not include test program output, but conveniently I observed the failure in my own setup:

Step 14/14 : RUN dunecontrol --opts=dune.opts --only=dune-istl bexec dune/istl/test/dotproducttest
 ---> Running in e9ba5bb79a3f
----- using default flags $CMAKE_FLAGS from /duneci/dune.opts -----
--- going to build dune-istl  ---
WARNING: commandline parameters will overwrite setting in opts file "/duneci/dune.opts"
--- calling bexec for dune-istl ---
DotProductTest	 	 ( Dune::BlockVector<Dune::FieldVector<int, 5>, std::__1::allocator<Dune::FieldVector<int, 5> > > and 
 	 	 	   Dune::BlockVector<Dune::FieldVector<int, 5>, std::__1::allocator<Dune::FieldVector<int, 5> > > )

DotProductTest	 	 ( Dune::VariableBlockVector<Dune::FieldVector<int, 1>, std::__1::allocator<Dune::FieldVector<int, 1> > > and 
 	 	 	   Dune::VariableBlockVector<Dune::FieldVector<int, 1>, std::__1::allocator<Dune::FieldVector<int, 1> > > )

DotProductTest	 	 ( Dune::BlockVector<Dune::FieldVector<float, 5>, std::__1::allocator<Dune::FieldVector<float, 5> > > and 
 	 	 	   Dune::BlockVector<Dune::FieldVector<std::__1::complex<float>, 5>, std::__1::allocator<Dune::FieldVector<std::__1::complex<float>, 5> > > )

dotproducttest: /duneci/dune-istl/dune/istl/test/dotproducttest.cc:79: int DotProductTest(const size_t, const size_t) [RealBlockVector = Dune::BlockVector<Dune::FieldVector<float, 5>, std::__1::allocator<Dune::FieldVector<float, 5> > >, ComplexBlockVector = Dune::BlockVector<Dune::FieldVector<std::__1::complex<float>, 5>, std::__1::allocator<Dune::FieldVector<std::__1::complex<float>, 5> > >]: Assertion `std::abs(result-complexSign*ct(2)*ctlength)<= myEps' failed.
bash: line 1:   217 Aborted                 (core dumped) dune/istl/test/dotproducttest
--- Failed to build dune-istl ---
Terminating dunecontrol due to previous errors!