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Without this fix, (substitute-object-in-subtree #0=(#0# 'a) 'a)
would dump core, since the C code would recurse indefinitely through
the infinite structure.  This patch adds an argument to the function,
and renames it to lread--substitute-object-in-subtree as the function
is not general-purpose and should not be relied on by outside code.
See Bug#23660.
* src/intervals.c (traverse_intervals_noorder): ARG is now void *,
not Lisp_Object, so that callers need not cons unnecessarily.
All callers changed.  Also, remove related #if-0 code that was
“temporary” in the early 1990s and has not been compilable for
some time.
* src/lread.c (struct subst): New type, for substitution closure data.
(seen_list): Remove this static var, as this info is now part of
struct subst.  All uses removed.
(Flread__substitute_object_in_subtree): Rename from
Fsubstitute_object_in_subtree, and give it a 3rd arg so that it
doesn’t dump core when called from the top level with an
already-cyclic structure.  All callers changed.
(SUBSTITUTE): Remove.  All callers expanded and then simplified.
(substitute_object_recurse): Take a single argument SUBST rather
than a pair OBJECT and PLACEHOLDER, so that its address can be
passed around as part of a closure; this avoids the need for an
AUTO_CONS call.  All callers changed.  If the COMPLETED component
is t, treat every subobject as potentially circular.
(substitute_in_interval): Take a struct subst * rather than a
Lisp_Object, for the closure data.  All callers changed.
* test/src/lread-tests.el (lread-lread--substitute-object-in-subtree):
New test, to check that the core dump does not reoccur.