Make UNKNOWN into an actual pseudo-type.

Previously, type "unknown" was labeled as a base type in pg_type, which
perhaps had some sense to it because you were allowed to create tables with
unknown-type columns.  But now that we don't allow that, it makes more
sense to label it a pseudo-type.  This has the additional effects of
forbidding use of "unknown" as a domain base type, cast source or target
type, PL function argument or result type, or plpgsql local variable type;
all of which seem like good holes to plug.

Discussion: https://postgr.es/m/CAH2L28uwwbL9HUM-WR=hromW1Cvamkn7O-g8fPY2m=_7muJ0oA@mail.gmail.com

doc/src/sgml/datatype.sgml

@@ -4660,6 +4660,10 @@ SELECT * FROM pg_attribute
     <primary>internal</primary>
    </indexterm>
 
+   <indexterm zone="datatype-pseudo">
+    <primary>unknown</primary>
+   </indexterm>
+
    <indexterm zone="datatype-pseudo">
     <primary>opaque</primary>
    </indexterm>
@@ -4781,9 +4785,16 @@ SELECT * FROM pg_attribute
         <entry>Indicates that a function returns no value.</entry>
        </row>
 
+       <row>
+        <entry><type>unknown</></entry>
+        <entry>Identifies a not-yet-resolved type, e.g. of an undecorated
+         string literal.</entry>
+       </row>
+
        <row>
         <entry><type>opaque</></entry>
-        <entry>An obsolete type name that formerly served all the above purposes.</entry>
+        <entry>An obsolete type name that formerly served many of the above
+         purposes.</entry>
        </row>
       </tbody>
      </tgroup>

doc/src/sgml/ddl.sgml

@@ -2579,7 +2579,7 @@ WHERE c.altitude > 500 AND c.tableoid = p.oid;
 
   <para>
    Another way to get the same effect is to use the <type>regclass</>
-   pseudo-type, which will print the table OID symbolically:
+   alias type, which will print the table OID symbolically:
 
 <programlisting>
 SELECT c.tableoid::regclass, c.name, c.altitude

doc/src/sgml/plhandler.sgml

@@ -26,7 +26,7 @@
     language such as C, using the version-1 interface, and registered
     with <productname>PostgreSQL</productname> as taking no arguments
     and returning the type <type>language_handler</type>.  This
-    special pseudotype identifies the function as a call handler and
+    special pseudo-type identifies the function as a call handler and
     prevents it from being called directly in SQL commands.
     For more details on C language calling conventions and dynamic loading,
     see <xref linkend="xfunc-c">.

doc/src/sgml/protocol.sgml

@@ -668,7 +668,7 @@
      number of parameter symbols (<literal>$</><replaceable>n</>)
      used in the query string.  Another special case is that a parameter's
      type can be specified as <type>void</> (that is, the OID of the
-     <type>void</> pseudotype).  This is meant to allow parameter symbols
+     <type>void</> pseudo-type).  This is meant to allow parameter symbols
      to be used for function parameters that are actually OUT parameters.
      Ordinarily there is no context in which a <type>void</> parameter
      could be used, but if such a parameter symbol appears in a function's

doc/src/sgml/queries.sgml

@@ -762,7 +762,7 @@ SELECT * FROM vw_getfoo;
      In some cases it is useful to define table functions that can
      return different column sets depending on how they are invoked.
      To support this, the table function can be declared as returning
-     the pseudotype <type>record</>.  When such a function is used in
+     the pseudo-type <type>record</>.  When such a function is used in
      a query, the expected row structure must be specified in the
      query itself, so that the system can know how to parse and plan
      the query.  This syntax looks like:

doc/src/sgml/ref/create_function.sgml

@@ -160,8 +160,8 @@ CREATE [ OR REPLACE ] FUNCTION
       </para>
       <para>
        Depending on the implementation language it might also be allowed
-       to specify <quote>pseudotypes</> such as <type>cstring</>.
-       Pseudotypes indicate that the actual argument type is either
+       to specify <quote>pseudo-types</> such as <type>cstring</>.
+       Pseudo-types indicate that the actual argument type is either
        incompletely specified, or outside the set of ordinary SQL data types.
       </para>
       <para>
@@ -199,7 +199,7 @@ CREATE [ OR REPLACE ] FUNCTION
        can be a base, composite, or domain type,
        or can reference the type of a table column.
        Depending on the implementation language it might also be allowed
-       to specify <quote>pseudotypes</> such as <type>cstring</>.
+       to specify <quote>pseudo-types</> such as <type>cstring</>.
        If the function is not supposed to return a value, specify
        <type>void</> as the return type.
       </para>

doc/src/sgml/ref/create_type.sgml

@@ -824,7 +824,7 @@ CREATE TYPE <replaceable class="parameter">name</replaceable>
    In <productname>PostgreSQL</productname> versions before 7.3, it
    was customary to avoid creating a shell type at all, by replacing the
    functions' forward references to the type name with the placeholder
-   pseudotype <type>opaque</>.  The <type>cstring</> arguments and
+   pseudo-type <type>opaque</>.  The <type>cstring</> arguments and
    results also had to be declared as <type>opaque</> before 7.3.  To
    support loading of old dump files, <command>CREATE TYPE</> will
    accept I/O functions declared using <type>opaque</>, but it will issue

src/backend/catalog/heap.c

@@ -490,8 +490,7 @@ CheckAttributeType(const char *attname,
 	char		att_typtype = get_typtype(atttypid);
 	Oid			att_typelem;
 
-	if (atttypid == UNKNOWNOID ||
-		att_typtype == TYPTYPE_PSEUDO)
+	if (att_typtype == TYPTYPE_PSEUDO)
 	{
 		/*
 		 * Refuse any attempt to create a pseudo-type column, except for a

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201701201
+#define CATALOG_VERSION_NO	201701251
 
 #endif

src/include/catalog/pg_type.h

@@ -418,7 +418,7 @@ DESCR("relative, limited-range time interval (Unix delta time)");
 DATA(insert OID = 704 (  tinterval PGNSP PGUID 12 f b T f t \054 0	 0 1025 tintervalin tintervalout tintervalrecv tintervalsend - - - i p f 0 -1 0 0 _null_ _null_ _null_ ));
 DESCR("(abstime,abstime), time interval");
 #define TINTERVALOID	704
-DATA(insert OID = 705 (  unknown   PGNSP PGUID -2 f b X f t \054 0	 0 0 unknownin unknownout unknownrecv unknownsend - - - c p f 0 -1 0 0 _null_ _null_ _null_ ));
+DATA(insert OID = 705 (  unknown   PGNSP PGUID -2 f p X f t \054 0	 0 0 unknownin unknownout unknownrecv unknownsend - - - c p f 0 -1 0 0 _null_ _null_ _null_ ));
 DESCR("");
 #define UNKNOWNOID		705
 

src/test/regress/expected/type_sanity.out

@@ -59,7 +59,7 @@ WHERE (p1.typtype = 'c' AND p1.typrelid = 0) OR
 -- Look for types that should have an array type according to their typtype,
 -- but don't.  We exclude composites here because we have not bothered to
 -- make array types corresponding to the system catalogs' rowtypes.
--- NOTE: as of 9.1, this check finds pg_node_tree, smgr, and unknown.
+-- NOTE: as of v10, this check finds pg_node_tree and smgr.
 SELECT p1.oid, p1.typname
 FROM pg_type as p1
 WHERE p1.typtype not in ('c','d','p') AND p1.typname NOT LIKE E'\\_%'
@@ -71,8 +71,7 @@ WHERE p1.typtype not in ('c','d','p') AND p1.typname NOT LIKE E'\\_%'
 -----+--------------
  194 | pg_node_tree
  210 | smgr
- 705 | unknown
-(3 rows)
+(2 rows)
 
 -- Make sure typarray points to a varlena array type of our own base
 SELECT p1.oid, p1.typname as basetype, p2.typname as arraytype,

src/test/regress/sql/type_sanity.sql

@@ -53,7 +53,7 @@ WHERE (p1.typtype = 'c' AND p1.typrelid = 0) OR
 -- Look for types that should have an array type according to their typtype,
 -- but don't.  We exclude composites here because we have not bothered to
 -- make array types corresponding to the system catalogs' rowtypes.
--- NOTE: as of 9.1, this check finds pg_node_tree, smgr, and unknown.
+-- NOTE: as of v10, this check finds pg_node_tree and smgr.
 
 SELECT p1.oid, p1.typname
 FROM pg_type as p1