Fix array slicing of int2vector and oidvector values.

The previous coding labeled expressions such as pg_index.indkey[1:3] as
being of int2vector type; which is not right because the subscript bounds
of such a result don't, in general, satisfy the restrictions of int2vector.
To fix, implicitly promote the result of slicing int2vector to int2[],
or oidvector to oid[].  This is similar to what we've done with domains
over arrays, which is a good analogy because these types are very much
like restricted domains of the corresponding regular-array types.

A side-effect is that we now also forbid array-element updates on such
columns, eg while "update pg_index set indkey[4] = 42" would have worked
before if you were superuser (and corrupted your catalogs irretrievably,
no doubt) it's now disallowed.  This seems like a good thing since, again,
some choices of subscripting would've led to results not satisfying the
restrictions of int2vector.  The case of an array-slice update was
rejected before, though with a different error message than you get now.
We could make these cases work in future if we added a cast from int2[]
to int2vector (with a cast function checking the subscript restrictions)
but it seems unlikely that there's any value in that.

Per report from Ronan Dunklau.  Back-patch to all supported branches
because of the crash risks involved.

src/backend/parser/parse_target.c

@@ -839,18 +839,20 @@ transformAssignmentSubscripts(ParseState *pstate,
 	/* If target was a domain over array, need to coerce up to the domain */
 	if (arrayType != targetTypeId)
 	{
+		Oid			resulttype = exprType(result);
+
 		result = coerce_to_target_type(pstate,
-									   result, exprType(result),
+									   result, resulttype,
 									   targetTypeId, targetTypMod,
 									   COERCION_ASSIGNMENT,
 									   COERCE_IMPLICIT_CAST,
 									   -1);
-		/* probably shouldn't fail, but check */
+		/* can fail if we had int2vector/oidvector, but not for true domains */
 		if (result == NULL)
 			ereport(ERROR,
 					(errcode(ERRCODE_CANNOT_COERCE),
 					 errmsg("cannot cast type %s to %s",
-							format_type_be(exprType(result)),
+							format_type_be(resulttype),
 							format_type_be(targetTypeId)),
 					 parser_errposition(pstate, location)));
 	}