BUG #17113: Assert failed on calling a function fixed after an extension reload

The following bug has been logged on the website:

Bug reference:      17113
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.com
PostgreSQL version: 13.3
Operating system:   Ubuntu 20.04
Description:

The following SQL snippet:
CREATE EXTENSION cube;
CREATE FUNCTION f1(c1 text, c2 text) RETURNS float8 AS $$
  SELECT cube_distance(cube(c1::text), cube(c2::text));
$$ LANGUAGE sql;

CREATE FUNCTION f2(c1 text, c2 text) RETURNS float8 AS $$
DECLARE
  res float8;
BEGIN
  SELECT f1(c1, c2) INTO res;
  RETURN res;
END;
$$ LANGUAGE plpgsql;

DROP EXTENSION cube;
SELECT f2('(0)', '(1)');
CREATE EXTENSION cube;
SELECT f2('(0)', '(1)');

leads to a failed assertion with the following stack:

Core was generated by `postgres: law regression [local] SELECT
                        '.
Program terminated with signal SIGABRT, Aborted.
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
50      ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb) bt
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#1  0x00007fc7d3326859 in __GI_abort () at abort.c:79
#2  0x000055a76178b342 in ExceptionalCondition
(conditionName=conditionName@entry=0x7fc7c41286e3 "!stmt->mod_stmt", 
    errorType=errorType@entry=0x7fc7c4126015 "FailedAssertion",
fileName=0x7ffce51ce2a0 "#\263xa\247U", 
    fileName@entry=0x7fc7c4128348 "pl_exec.c",
lineNumber=lineNumber@entry=4225) at assert.c:69
#3  0x00007fc7c4118111 in exec_stmt_execsql
(estate=estate@entry=0x7ffce51ce850, stmt=stmt@entry=0x55a763a29210)
    at pl_exec.c:4225
#4  0x00007fc7c41191e0 in exec_stmts (estate=estate@entry=0x7ffce51ce850,
stmts=0x55a763a29268) at pl_exec.c:2059
#5  0x00007fc7c41198a8 in exec_stmt_block
(estate=estate@entry=0x7ffce51ce850, block=block@entry=0x55a763a29340)
    at pl_exec.c:1910
#6  0x00007fc7c411998e in exec_toplevel_block
(estate=estate@entry=0x7ffce51ce850, block=0x55a763a29340)
    at pl_exec.c:1608
#7  0x00007fc7c4119c88 in plpgsql_exec_function
(func=func@entry=0x55a763a76da8, fcinfo=fcinfo@entry=0x55a763a9f0a8, 
    simple_eval_estate=simple_eval_estate@entry=0x0,
simple_eval_resowner=simple_eval_resowner@entry=0x0, 
    procedure_resowner=procedure_resowner@entry=0x0, atomic=<optimized out>)
at pl_exec.c:611
#8  0x00007fc7c4124802 in plpgsql_call_handler (fcinfo=0x55a763a9f0a8) at
pl_handler.c:277
#9  0x000055a761487629 in ExecInterpExpr (state=0x55a763a9efc0,
econtext=0x55a763a9ed00, isnull=0x7ffce51ceb67)
    at execExprInterp.c:725
#10 0x000055a761483f07 in ExecInterpExprStillValid (state=0x55a763a9efc0,
econtext=0x55a763a9ed00, 
    isNull=0x7ffce51ceb67) at execExprInterp.c:1824
#11 0x000055a7614c39d4 in ExecEvalExprSwitchContext (isNull=0x7ffce51ceb67,
econtext=0x55a763a9ed00, 
    state=0x55a763a9efc0) at ../../../src/include/executor/executor.h:339
#12 ExecProject (projInfo=0x55a763a9efb8) at
../../../src/include/executor/executor.h:373
#13 ExecResult (pstate=<optimized out>) at nodeResult.c:136
#14 0x000055a76149416c in ExecProcNodeFirst (node=0x55a763a9ebe8) at
execProcnode.c:463
#15 0x000055a76148cdbc in ExecProcNode (node=0x55a763a9ebe8) at
../../../src/include/executor/executor.h:257
#16 ExecutePlan (estate=estate@entry=0x55a763a9e9b0,
planstate=0x55a763a9ebe8, use_parallel_mode=<optimized out>, 
    operation=operation@entry=CMD_SELECT, sendTuples=sendTuples@entry=true,
numberTuples=numberTuples@entry=0, 
    direction=ForwardScanDirection, dest=0x55a763a2d3b8, execute_once=true)
at execMain.c:1551
#17 0x000055a76148cf9c in standard_ExecutorRun (queryDesc=0x55a76395adc0,
direction=ForwardScanDirection, count=0, 
    execute_once=<optimized out>) at execMain.c:361
#18 0x000055a76148d068 in ExecutorRun
(queryDesc=queryDesc@entry=0x55a76395adc0, 
    direction=direction@entry=ForwardScanDirection, count=count@entry=0,
execute_once=<optimized out>) at execMain.c:305
#19 0x000055a76164d2f8 in PortalRunSelect
(portal=portal@entry=0x55a7639a9bb0, forward=forward@entry=true, count=0, 
    count@entry=9223372036854775807, dest=dest@entry=0x55a763a2d3b8) at
pquery.c:919
#20 0x000055a76164ecbb in PortalRun (portal=portal@entry=0x55a7639a9bb0,
count=count@entry=9223372036854775807, 
    isTopLevel=isTopLevel@entry=true, run_once=run_once@entry=true,
dest=dest@entry=0x55a763a2d3b8, 
    altdest=altdest@entry=0x55a763a2d3b8, qc=0x7ffce51cee70) at
pquery.c:763
#21 0x000055a76164ada7 in exec_simple_query
(query_string=query_string@entry=0x55a7639394b0 "SELECT f2('(0)',
'(1)');")
    at postgres.c:1214
#22 0x000055a76164cd79 in PostgresMain (argc=argc@entry=1,
argv=argv@entry=0x7ffce51cf060, dbname=<optimized out>, 
    username=<optimized out>) at postgres.c:4486
#23 0x000055a7615a62f3 in BackendRun (port=port@entry=0x55a76395a0b0) at
postmaster.c:4506
#24 0x000055a7615a9508 in BackendStartup (port=port@entry=0x55a76395a0b0) at
postmaster.c:4228
#25 0x000055a7615a974f in ServerLoop () at postmaster.c:1745
#26 0x000055a7615aac9c in PostmasterMain (argc=3, argv=<optimized out>) at
postmaster.c:1417
#27 0x000055a7614eb752 in main (argc=3, argv=0x55a763933520) at main.c:209

reproduced on REL9_6_STABLE..master.