Baffling error message and code

[Steve Teale <steve.teale@xxxxxxxxxxxxxxxx>]

As I've said before, I am trying to get the GDC implementation that
worked with 4.3, working with 4.4.

The function it is compiling is to my mind not very well written - kind
of mixed up. But the error message is even more confusing:

-c ../../../libphobos/std/stream.d
In file included from <builtin>:2:
../../../libphobos/std/stream.d: In member function ‘vreadf’:
../../../libphobos/std/stream.d:692: internal compiler error: in
insert_into_preds_of_block, at tree-ssa-pre.c:3114

The offending code in tree-ssa-pre.c is:

  FOR_EACH_EDGE (pred, ei, block->preds)
    {
      pre_expr ae = avail[pred->src->index];
      gcc_assert (get_expr_type (ae) == type ||
          useless_type_conversion_p (type, get_expr_type (ae)));

If anyone has any understanding of what is happening here and what the
assert is intended to prevent, your help would be most welcome.

Thanks
Steve