[Patch, fortran] PR59198 - [4.7/4.8/4.9 Regression] ICE on cyclically dependent polymorphic types

[Paul Richard Thomas <paul dot richard dot thomas at gmail dot com>]

Dear All,

The attached patch allows the original testcase to compile but
attempting to use it runs into segfaults at runtime.  The initializer
for 'decay_t' has a bad FIELD_DECL for 'decay_gen_t' since it lacks
the DECL_SIZE(_UNIT) fields but the TYPE_SIZE(_UNIT) fields are OK.
The patch runs through the constructor and restores the missing
fields, thus preventing the ICE in varasm.c.

Taking the reduced testcase of comment #4 and adding a main program:

module decays

  implicit none

  interface
    real elemental function iface (arg)
      real, intent(in) :: arg
    end function
  end interface

  type :: decay_term_t
     type(decay_t), pointer :: unstable_product
  end type

  type :: decay_gen_t
     procedure(iface), nopass, pointer :: obs1_int
     type(decay_term_t), allocatable :: term
  end type

  type :: rng_t
    integer :: i
  end type

  type, extends (decay_gen_t) :: decay_t
     class(rng_t), allocatable :: rng
  end type

  class(decay_t), allocatable :: object

end

  use decays
  type(decay_t), pointer :: template
  allocate (template)
  allocate (template%rng)
  template%obs1_int => cos
  print *, template%obs1_int (3.14159/2.)
  allocate (object, source = template)
!  print *, object%obs1_int (3.14159/2.) ! This should be OK but it
segfaults at runtime.
  object%obs1_int => sin
  print *, object%obs1_int (3.14159/2.)
end

This runs correctly.  However, the commented out line causes a
segfault in runtime.  I haven't attempted yet to understand why this
should be the case but it is obvious that the _copy procedure for
decay_t has something wrong with it.

The patch, therefore is neither ready in principle to be committed and
uncouvers other nasties.  I have two questions that I hope that
somebody can answer:
(i) How or why is it possible for build_constructor to produce
incomplete FIELD_DECLS?; and
(ii) What is wrong with the _copy procedure?

I cannot return to the fray for slightly more than a week.

Best regards

Paul

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