CREATE INDEX with an expression in an INCLUDE column fails with XX000 "unrecognized node type" instead of 0A000 on master

PostgreSQL version: 20devel (git master)

On current master, creating an index with an expression in an INCLUDE
(non-key) column fails with an internal error ("unrecognized node
type", SQLSTATE XX000) instead of the user-facing
FEATURE_NOT_SUPPORTED (0A000) "expressions are not supported in
included columns". The statement is still correctly rejected, but with
the wrong error class and a message.

Minimal repro

CREATE TABLE foo (id int PRIMARY KEY, x int, y int);
CREATE INDEX idx_foo ON foo (x) INCLUDE ((x + y));

Actual result on master (20devel):

postgres=# select version();
version
------------------------------------------------------------------------
PostgreSQL 20devel on aarch64-unknown-linux-gnu, compiled by gcc
(GCC) 7.3.1 20180712 (Red Hat 7.3.1-18), 64-bit
(1 row)

postgres=# CREATE TABLE foo (id int PRIMARY KEY, x int, y int);
CREATE TABLE
postgres=# CREATE INDEX idx_foo ON foo (x) INCLUDE ((x + y));
ERROR: unrecognized node type: 74
postgres=# \errverbose
ERROR: XX000: unrecognized node type: 74
LOCATION: expression_tree_walker_impl, nodeFuncs.c:2733

(built from git master, HEAD at the time of testing:
572c3b2ddf8c90303d57bf33add4dcbf1b30b866)

Expected result (behavior on all released majors, e.g. 16.13):

postgres=# CREATE INDEX idx_foo ON foo (x) INCLUDE ((x + y));
ERROR: expressions are not supported in included columns
postgres=# \errverbose
ERROR: 0A000: expressions are not supported in included columns
LOCATION: ComputeIndexAttrs, indexcmds.c:1910

This appears to have been introduced by:

commit 181b6185c79e09e6ac94428189d9afac807244ac
"Improve the names generated for indexes on expressions"
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=181b6185c79e09e6ac94428189d9afac807244ac

which replaced the previous static "expr" fallback name with a walk of
the expression. Prior to that commit the INCLUDE expression was never
walked at this stage.

The trigger is any parenthesized expression in INCLUDE, not just
arithmetic; e.g. INCLUDE ((x)) fails the same way with "unrecognized
node type: 72"

(CC'ing Tom Lane, author of the patch that I've linked which is likely
causing the issue, and Matthew Ripley, author of a test that led me
down to this discovery when run against master)

Regards,
Maaz Syed Adeeb