$> cat reproducer.f90
  subroutine TestOfCharacter(a0, a1)
    character(len=*), intent(in) :: a0
    character(len=*), intent(inout):: a1

    !$omp target map(to:a0) map(from: a1)
    a1 = a0
    !$omp end target
  end subroutine TestOfCharacter
$> flang-new -fc1 -emit-hlfir  -fopenmp reproducer.f90
flang-new: /work1/omp-nightly/build/git/trunk21.0/llvm-project/flang/lib/Lower/OpenMP/OpenMP.cpp:1374: void genBodyOfTargetOp(Fortran::lower::AbstractConverter&, Fortran::lower::SymMap&, Fortran::semantics::SemanticsContext&, Fort\
ran::lower::pft::Evaluation&, mlir::omp::TargetOp&, const Fortran::common::openmp::EntryBlockArgs&, const mlir::Location&, const ConstructQueue&, llvm::SmallVectorImpl<tomp::DirectiveWithClauses<Fortran::lower::omp::Clause> >::con\
st_iterator, Fortran::lower::omp::DataSharingProcessor&): Assertion `valOp != nullptr' failed.
 

$> flang-new --version
flang version 21.0.0git (https://github.com/llvm/llvm-project a168ddc470825091ad52da12042fb38491ed81d6)
Target: x86_64-unknown-linux-gnu