valid casts to anyarray

Hi list

I've been down a rabbit hole today trying to understand what exactly makes
a type a valid candidate for an ANYARRAY function argument (e.g., something
you can 'unnest()').

My reading has led me across such functions as 'get_promoted_array_type',
'IsTrueArrayType', 'can_coerce_type', and 'check_generic_type_consistency',
and this has led me to believe that any type which has a valid (i.e.,
non-zero?) pg_type.typelem defined should be applicable.

However, I cannot seem to be able to call 'unnest' on a 'point':

postgres=# select unnest(point(1,2));
ERROR: function unnest(point) does not exist

... even though according to 'pg_catalog.pg_type' the type 'point' does
indeed look very array-like (it should be equivalent to an float8 array).
The only difference I can spot is that it has
'typsubscript=raw_array_subscript_handler', as opposed to
typsubscript=array_subscript_handler' which is what 'IsTrueArrayType'
checks for.

Can anyone here perhaps enlighten me as to how I can tell if a type is a
valid ANYARRAY (and bonus points to point out the check I must have missed
in the (parser?) source code)?

-Philip