argtype_inherit() is dead code

[Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>]

In parse_func.c there are routines argtype_inherit() and
gen_cross_product() that date from Berkeley days.  The comments
explain their reason for existence thus:

 *  This function is used to handle resolution of function calls when
 *  there is no match to the given argument types, but there might be
 *  matches based on considering complex types as members of their
 *  superclass types (parent classes).
 *
 *  It takes an array of input type ids.  For each type id in the array
 *  that's a complex type (a class), it walks up the inheritance tree,
 *  finding all superclasses of that type. A vector of new Oid type
 *  arrays is returned to the caller, listing possible alternative
 *  interpretations of the input typeids as members of their superclasses
 *  rather than the actually given argument types. The vector is
 *  terminated by a NULL pointer.
 *
 *  The order of this vector is as follows:  all superclasses of the
 *  rightmost complex class are explored first.  The exploration
 *  continues from right to left.  This policy means that we favor
 *  keeping the leftmost argument type as low in the inheritance tree
 *  as possible.  This is intentional; it is exactly what we need to
 *  do for method dispatch.
 *
 *  The vector does not include the case where no complex classes have
 *  been promoted, since that was already tried before this routine
 *  got called.

I realized that this is effectively dead code: although it can be
executed, it can never produce any useful results.  The reason is that
can_coerce_type() already knows that inherited rowtypes can be promoted
to their parent rowtypes, and it considers that a legal implicit
coercion.  This means that any possible function matches based on
promoting child rowtypes to ancestors were found in func_get_detail()'s
first pass.  If there is exactly one match then it will be taken as
the correct answer and returned without calling argtype_inherit().
If there is more than one match then func_get_detail() will fail
(return FUNCDETAIL_MULTIPLE), again without calling argtype_inherit().
The only way to reach argtype_inherit() is if there are *no* ancestor
matches, which means that the function is a very expensive no-op that
we execute just before throwing an error.

I'm strongly tempted to just rip out argtype_inherit() and
gen_cross_product().  Even if we suppose that we might want to resurrect
the claimed functionality someday, I don't think it could be made to
work this way.  You'd have to put the knowledge into
func_select_candidate() instead, else there'd be very weird interactions
with the heuristics for resolving non-complex input argument types.

Thoughts?

			regards, tom lane