[BUGS] BUG #14590: something wrong when create extension in schema pg_catalog

The following bug has been logged on the website:

Bug reference:      14590
Logged by:          Zhou Digoal
Email address:      digoal@126.com
PostgreSQL version: 9.6.2
Operating system:   CentOS 6.x x64
Description:


```  
cd $PGHOME/share/extension  
  
cp btree_gist--1.2.sql btree_gist--9.9.9.sql  
  
vi btree_gist--9.9.9.sql  
-- add this
create type new_type as (  
  id int,  
  c1 int  
);  
```  
  
test
  
```  
postgres=# create extension btree_gist with schema pg_catalog;  
CREATE EXTENSION  
postgres=# drop extension btree_gist ;  
DROP EXTENSION  
  


postgres=# \set VERBOSITY verbose  
postgres=# create extension btree_gist with schema pg_catalog version
'9.9.9';  
ERROR:  42501: permission denied to create "pg_catalog.new_type"  
DETAIL:  System catalog modifications are currently disallowed.  
LOCATION:  heap_create, heap.c:280  
```  
  
but why can do this?
  
```  
postgres=# create extension btree_gist version '9.9.9';;  
CREATE EXTENSION  
postgres=# alter extension btree_gist set schema pg_catalog;  
ALTER EXTENSION  
```  
  
and why can do this?
  
```   
vi $PGDATA/postgresql.conf  
allow_system_table_mods=on  
  
pg_ctl restart -m fast  
  

postgres=# create extension btree_gist with schema pg_catalog version
'9.9.9';  
CREATE EXTENSION  
```  
  
table, composite type, seq ... in extension sql, so cann't create in
pg_catalog?

it's a bug?

best regards
digoal


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