BUG #18363: Assert !ReindexIsProcessingIndex falsified with expression index over select from table

The following bug has been logged on the website:

Bug reference:      18363
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.com
PostgreSQL version: 16.2
Operating system:   Ubuntu 22.04
Description:

The following script:
CREATE TABLE t(i INT PRIMARY KEY);
CREATE FUNCTION f(i int) RETURNS int IMMUTABLE LANGUAGE SQL
  RETURN (SELECT i FROM t WHERE i = $1);
CREATE INDEX ON t(f(i));
INSERT INTO t VALUES (1);
REINDEX INDEX t_f_idx;

triggers an assertion failure with the stack trace:
TRAP: failed
Assert("!ReindexIsProcessingIndex(RelationGetRelid(indexRelation))"), File:
"indexam.c", Line: 778, PID: 1339223
ExceptionalCondition at assert.c:52:13
index_getprocid at indexam.c:817:1
get_relation_info at plancat.c:272:24
build_simple_rel at relnode.c:379:5
add_base_rels_to_query at initsplan.c:166:10
add_base_rels_to_query at initsplan.c:173:3
query_planner at planmain.c:180:2
grouping_planner at planner.c:1495:17
subquery_planner at planner.c:1070:2
make_subplan at subselect.c:221:12
process_sublinks_mutator at subselect.c:1950:10
expression_tree_mutator_impl at nodeFuncs.c:3302:5
process_sublinks_mutator at subselect.c:2052:9
expression_tree_mutator_impl at nodeFuncs.c:3389:12
process_sublinks_mutator at subselect.c:2052:9
SS_process_sublinks at subselect.c:1924:1
preprocess_expression at planner.c:1171:10
subquery_planner at planner.c:810:20
standard_planner at planner.c:413:9
planner at planner.c:282:9
pg_plan_query at postgres.c:904:9
init_execution_state at functions.c:497:12
init_sql_fcache at functions.c:797:21
fmgr_sql at functions.c:1097:40
ExecInterpExpr at execExprInterp.c:735:7
ExecInterpExprStillValid at execExprInterp.c:1871:1
MemoryContextSwitchTo at palloc.h:142:23
 (inlined by) ExecEvalExprSwitchContext at executor.h:356:2
 (inlined by) FormIndexDatum at index.c:2774:13
heapam_index_build_range_scan at heapam_handler.c:1660:7
table_index_build_scan at tableam.h:1781:9
 (inlined by) _bt_spools_heapscan at nbtsort.c:483:15
btbuild at nbtsort.c:329:14
index_build at index.c:3042:10
reindex_index at index.c:3763:2
ReindexIndex at indexcmds.c:2793:1
ExecReindex at indexcmds.c:2741:1
standard_ProcessUtility at utility.c:965:4
ProcessUtility at utility.c:530:3
PortalRunUtility at pquery.c:1168:2
PortalRunMulti at pquery.c:1315:5
PortalRun at pquery.c:795:5
exec_simple_query at postgres.c:1282:3
PostgresMain at postgres.c:4641:27
report_fork_failure_to_client at postmaster.c:4242:1
BackendStartup at postmaster.c:4199:22
ServerLoop at postmaster.c:1788:6
BackgroundWorkerInitializeConnection at postmaster.c:5604:1
main at main.c:185:3

With a non-assertion-enabled build it fails with:
ERROR:  could not read block 0 in file "base/16384/16392": read only 0 of
8192 bytes
CONTEXT:  SQL function "f" during startup

Surely, such an index will not work correctly anyway, but may be it makes
sense to replace that Assert with ereport(ERROR).