https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100651
--- Comment #2 from G. Steinmetz <gs...@t-online.de> --- Simplified a bit : $ cat z1.f90 module m type :: t contains procedure, pass(this) :: assign_to_string generic :: assignment(=) => assign_to_string end type contains subroutine assign_to_string(string, this) character(:), allocatable, intent(out) :: string class(t), intent(in) :: this end subroutine assert_code(err_msg) character(:), optional, allocatable, intent(inout) :: err_msg err_msg = 'foo bar' end function to_int(err_msg) result(ptr) integer :: ptr character(:), optional, allocatable, intent(inout) :: err_msg ptr = 1 call assert_code(err_msg) end end program main use m type, extends(t) :: t2 real(8) :: value = 1 end type type(t2) :: x integer :: n character(:), allocatable :: err_msg x = t2(2) n = to_int(err_msg) print *, 'allocated :', allocated(err_msg) print *, 'len : ', len(err_msg) ! print *, 'len_trim : ', len_trim(err_msg) print *, 'err_msg : ', err_msg end $ cat z2.f90 module m type :: t contains procedure, pass(this) :: assign_to_string generic :: assignment(=) => assign_to_string end type contains subroutine assign_to_string(string, this) character(:), allocatable, intent(out) :: string class(t), intent(in) :: this end subroutine assert_code(err_msg) character(:), allocatable, intent(inout) :: err_msg err_msg = 'foo bar' end function to_int(err_msg) result(ptr) integer :: ptr character(:), allocatable, intent(inout) :: err_msg ptr = 1 call assert_code(err_msg) end end program main use m type, extends(t) :: t2 real(8) :: value = 1 end type type(t2) :: x integer :: n character(:), allocatable :: err_msg x = t2(2) n = to_int(err_msg) print *, 'allocated :', allocated(err_msg) print *, 'len : ', len(err_msg) ! print *, 'len_trim : ', len_trim(err_msg) print *, 'err_msg : ', err_msg end $ gfortran-12-20210516 z1.f90 -g -O0 && ./a.out allocated : T len : 0 Operating system error: Cannot allocate memory Memory allocation failure in xrealloc Error termination. Backtrace: #0 0x40d1c7 in write_character at ../../../libgfortran/io/write.c:1416 #1 0x41222e in list_formatted_write_scalar at ../../../libgfortran/io/write.c:1900 #2 0x40265e in MAIN__ at .../z1.f90:36 #3 0x4026a4 in main at .../z1.f90:24 $ gfortran-12-20210516 z2.f90 -g -O0 && ./a.out allocated : T len : 7 err_msg : foo bar