Support domains over composite types.

This is the last major omission in our domains feature: you can now
make a domain over anything that's not a pseudotype.

The major complication from an implementation standpoint is that places
that might be creating tuples of a domain type now need to be prepared
to apply domain_check().  It seems better that unprepared code fail
with an error like "<type> is not composite" than that it silently fail
to apply domain constraints.  Therefore, relevant infrastructure like
get_func_result_type() and lookup_rowtype_tupdesc() has been adjusted
to treat domain-over-composite as a distinct case that unprepared code
won't recognize, rather than just transparently treating it the same
as plain composite.  This isn't a 100% solution to the possibility of
overlooked domain checks, but it catches most places.

In passing, improve typcache.c's support for domains (it can now cache
the identity of a domain's base type), and rewrite the argument handling
logic in jsonfuncs.c's populate_record[set]_worker to reduce duplicative
per-call lookups.

I believe this is code-complete so far as the core and contrib code go.
The PLs need varying amounts of work, which will be tackled in followup
patches.

Discussion: https://postgr.es/m/4206.1499798337@sss.pgh.pa.us

doc/src/sgml/datatype.sgml

@@ -4379,8 +4379,7 @@ SET xmloption TO { DOCUMENT | CONTENT };
     underlying type — for example, any operator or function that
     can be applied to the underlying type will work on the domain type.
     The underlying type can be any built-in or user-defined base type,
-    enum type, array or range type, or another domain.  (Currently, domains
-    over composite types are not implemented.)
+    enum type, array type, composite type, range type, or another domain.
    </para>
 
    <para>

doc/src/sgml/rowtypes.sgml

@@ -84,8 +84,9 @@ CREATE TABLE inventory_item (
   restriction of the current implementation: since no constraints are
   associated with a composite type, the constraints shown in the table
   definition <emphasis>do not apply</emphasis> to values of the composite type
-  outside the table.  (A partial workaround is to use domain
-  types as members of composite types.)
+  outside the table.  (To work around this, create a domain over the composite
+  type, and apply the desired constraints as <literal>CHECK</literal>
+  constraints of the domain.)
  </para>
  </sect2>
 

doc/src/sgml/xfunc.sgml

@@ -351,6 +351,31 @@ CREATE FUNCTION tf1 (accountno integer, debit numeric) RETURNS numeric AS $$
         WHERE accountno = tf1.accountno
     RETURNING balance;
 $$ LANGUAGE SQL;
+</programlisting>
+    </para>
+
+    <para>
+     A <acronym>SQL</acronym> function must return exactly its declared
+     result type.  This may require inserting an explicit cast.
+     For example, suppose we wanted the
+     previous <function>add_em</function> function to return
+     type <type>float8</type> instead.  This won't work:
+
+<programlisting>
+CREATE FUNCTION add_em(integer, integer) RETURNS float8 AS $$
+    SELECT $1 + $2;
+$$ LANGUAGE SQL;
+</programlisting>
+
+     even though in other contexts <productname>PostgreSQL</productname>
+     would be willing to insert an implicit cast to
+     convert <type>integer</type> to <type>float8</type>.
+     We need to write it as
+
+<programlisting>
+CREATE FUNCTION add_em(integer, integer) RETURNS float8 AS $$
+    SELECT ($1 + $2)::float8;
+$$ LANGUAGE SQL;
 </programlisting>
     </para>
    </sect2>
@@ -452,13 +477,16 @@ $$ LANGUAGE SQL;
       </listitem>
       <listitem>
        <para>
-        You must typecast the expressions to match the
-        definition of the composite type, or you will get errors like this:
+        We must ensure each expression's type matches the corresponding
+        column of the composite type, inserting a cast if necessary.
+        Otherwise we'll get errors like this:
 <screen>
 <computeroutput>
 ERROR:  function declared to return emp returns varchar instead of text at column 1
 </computeroutput>
 </screen>
+        As with the base-type case, the function will not insert any casts
+        automatically.
        </para>
       </listitem>
      </itemizedlist>
@@ -478,6 +506,11 @@ $$ LANGUAGE SQL;
      in this situation, but it is a handy alternative in some cases
      &mdash; for example, if we need to compute the result by calling
      another function that returns the desired composite value.
+     Another example is that if we are trying to write a function that
+     returns a domain over composite, rather than a plain composite type,
+     it is always necessary to write it as returning a single column,
+     since there is no other way to produce a value that is exactly of
+     the domain type.
     </para>
 
     <para>