Create a new type category for "internal use" types.

Historically we've put type "char" into the S (String) typcategory,
although calling it a string is a stretch considering it can only
store one byte.  (In our actual usage, it's more like an enum.)
This choice now seems wrong in view of the special heuristics
that parse_func.c and parse_coerce.c have for TYPCATEGORY_STRING:
it's not a great idea for "char" to have those preferential casting
behaviors.

Worse than that, recent patches inventing special-purpose types
like pg_node_tree have assigned typcategory S to those types,
meaning they also get preferential casting treatment that's designed
on the assumption that they can hold arbitrary text.

To fix, invent a new category TYPCATEGORY_INTERNAL for internal-use
types, and assign that to all these types.  I used code 'Z' for
lack of a better idea ('I' was already taken).

This change breaks one query in psql/describe.c, which now needs to
explicitly cast a catalog "char" column to text before concatenating
it with an undecorated literal.  Also, a test case in contrib/citext
now needs an explicit cast to convert citext to "char".  Since the
point of this change is to not have "char" be a surprisingly-available
cast target, these breakages seem OK.

Per report from Ian Campbell.

Discussion: https://postgr.es/m/2216388.1638480141@sss.pgh.pa.us

3 files changed, 14 insertions, 3 deletions

diff --git a/contrib/citext/expected/citext.out b/contrib/citext/expected/citext.out
index ec99aaed5d..3bac0534fb 100644
--- a/contrib/citext/expected/citext.out
+++ b/contrib/citext/expected/citext.out
@@ -1089,7 +1089,12 @@ INSERT INTO caster (char)          VALUES ('f'::citext);
 INSERT INTO caster (citext)        VALUES ('f'::char);
 INSERT INTO caster (chr)           VALUES ('f'::text);
 INSERT INTO caster (text)          VALUES ('f'::"char");
-INSERT INTO caster (chr)           VALUES ('f'::citext);
+INSERT INTO caster (chr)           VALUES ('f'::citext);  -- requires cast
+ERROR:  column "chr" is of type "char" but expression is of type citext
+LINE 1: INSERT INTO caster (chr)           VALUES ('f'::citext);
+                                                   ^
+HINT:  You will need to rewrite or cast the expression.
+INSERT INTO caster (chr)           VALUES ('f'::citext::text);
 INSERT INTO caster (citext)        VALUES ('f'::"char");
 INSERT INTO caster (name)          VALUES ('foo'::text);
 INSERT INTO caster (text)          VALUES ('foo'::name);
diff --git a/contrib/citext/expected/citext_1.out b/contrib/citext/expected/citext_1.out
index 75fd08b7cc..57fc863f7a 100644
--- a/contrib/citext/expected/citext_1.out
+++ b/contrib/citext/expected/citext_1.out
@@ -1089,7 +1089,12 @@ INSERT INTO caster (char)          VALUES ('f'::citext);
 INSERT INTO caster (citext)        VALUES ('f'::char);
 INSERT INTO caster (chr)           VALUES ('f'::text);
 INSERT INTO caster (text)          VALUES ('f'::"char");
-INSERT INTO caster (chr)           VALUES ('f'::citext);
+INSERT INTO caster (chr)           VALUES ('f'::citext);  -- requires cast
+ERROR:  column "chr" is of type "char" but expression is of type citext
+LINE 1: INSERT INTO caster (chr)           VALUES ('f'::citext);
+                                                   ^
+HINT:  You will need to rewrite or cast the expression.
+INSERT INTO caster (chr)           VALUES ('f'::citext::text);
 INSERT INTO caster (citext)        VALUES ('f'::"char");
 INSERT INTO caster (name)          VALUES ('foo'::text);
 INSERT INTO caster (text)          VALUES ('foo'::name);
diff --git a/contrib/citext/sql/citext.sql b/contrib/citext/sql/citext.sql
index 10232f5a9f..55fb1d11a6 100644
--- a/contrib/citext/sql/citext.sql
+++ b/contrib/citext/sql/citext.sql
@@ -361,7 +361,8 @@ INSERT INTO caster (citext)        VALUES ('f'::char);
 
 INSERT INTO caster (chr)           VALUES ('f'::text);
 INSERT INTO caster (text)          VALUES ('f'::"char");
-INSERT INTO caster (chr)           VALUES ('f'::citext);
+INSERT INTO caster (chr)           VALUES ('f'::citext);  -- requires cast
+INSERT INTO caster (chr)           VALUES ('f'::citext::text);
 INSERT INTO caster (citext)        VALUES ('f'::"char");
 
 INSERT INTO caster (name)          VALUES ('foo'::text);