Prevent invoking I/O conversion casts via functional/attribute notation.

PG 8.4 added a built-in feature for casting pretty much any data type to
string types (text, varchar, etc).  We allowed this to work in any of the
historically-allowed syntaxes: CAST(x AS text), x::text, text(x), or
x.text.  However, multiple complaints have shown that it's too easy to
invoke such casts unintentionally in the latter two styles, particularly
field selection.  To cure the problem with the narrowest possible change
of behavior, disallow use of I/O conversion casts from composite types to
string types via functional/attribute syntax.  The new functionality is
still available via cast syntax.

In passing, document the equivalence of functional and attribute syntax
in a more visible place.

doc/src/sgml/ref/create_cast.sgml

@@ -38,7 +38,7 @@ CREATE CAST (<replaceable>source_type</replaceable> AS <replaceable>target_type<
   <para>
    <command>CREATE CAST</command> defines a new cast.  A cast
    specifies how to perform a conversion between
-   two data types.  For example:
+   two data types.  For example,
 <programlisting>
 SELECT CAST(42 AS float8);
 </programlisting>
@@ -64,10 +64,13 @@ SELECT CAST(42 AS float8);
   </para>
 
   <para>
-   You can define a cast as an <firstterm>I/O conversion cast</> using
+   You can define a cast as an <firstterm>I/O conversion cast</> by using
    the <literal>WITH INOUT</literal> syntax. An I/O conversion cast is
    performed by invoking the output function of the source data type, and
-   passing the result to the input function of the target data type.
+   passing the resulting string to the input function of the target data type.
+   In many common cases, this feature avoids the need to write a separate
+   cast function for conversion. An I/O conversion cast acts the same as
+   a regular function-based cast; only the implementation is different.
   </para>
 
   <para>
@@ -218,7 +221,7 @@ SELECT CAST ( 2 AS numeric ) + 4.0;
       <para>
        Indicates that the cast is an I/O conversion cast, performed by
        invoking the output function of the source data type, and passing the
-       result to the input function of the target data type.
+       resulting string to the input function of the target data type.
       </para>
      </listitem>
     </varlistentry>
@@ -278,9 +281,9 @@ SELECT CAST ( 2 AS numeric ) + 4.0;
   <para>
    When a cast has different source and
    target types and a function that takes more than one argument, it
-   represents converting from one type to another and applying a length
+   supports converting from one type to another and applying a length
    coercion in a single step.  When no such entry is available, coercion
-   to a type that uses a type modifier involves two steps, one to
+   to a type that uses a type modifier involves two cast steps, one to
    convert between data types and a second to apply the modifier.
   </para>
 
@@ -366,6 +369,18 @@ SELECT CAST ( 2 AS numeric ) + 4.0;
     syntax.
    </para>
   </note>
+
+  <note>
+   <para>
+    There's an exception to the exception, too: I/O conversion casts from
+    composite types to string types cannot be invoked using functional
+    syntax, but must be written in explicit cast syntax (either
+    <literal>CAST</> or <literal>::</> notation).  This exception was added
+    because after the introduction of automatically-provided I/O conversion
+    casts, it was found too easy to accidentally invoke such a cast when
+    a function or column reference was intended.
+   </para>
+  </note>
  </refsect1>
 
 

doc/src/sgml/syntax.sgml

@@ -1522,6 +1522,19 @@ sqrt(2)
     The arguments can optionally have names attached.
     See <xref linkend="sql-syntax-calling-funcs"> for details.
    </para>
+
+   <note>
+    <para>
+     A function that takes a single argument of composite type can
+     optionally be called using field-selection syntax, and conversely
+     field selection can be written in functional style.  That is, the
+     notations <literal>col(table)</> and <literal>table.col</> are
+     interchangeable.  This behavior is not SQL-standard but is provided
+     in <productname>PostgreSQL</> because it allows use of functions to
+     emulate <quote>computed fields</>.  For more information see
+     <xref linkend="xfunc-sql-composite-functions">.
+    </para>
+   </note>
   </sect2>
 
   <sect2 id="syntax-aggregates">

doc/src/sgml/xfunc.sgml

@@ -271,7 +271,7 @@ $$ LANGUAGE SQL;
     </para>
    </sect2>
 
-   <sect2>
+   <sect2 id="xfunc-sql-composite-functions">
     <title><acronym>SQL</acronym> Functions on Composite Types</title>
 
     <para>
@@ -492,6 +492,12 @@ SELECT emp.name, emp.double_salary FROM emp;
       <literal>double_salary</> isn't a real column of the table.
       (You can also emulate computed fields with views.)
      </para>
+
+     <para>
+      Because of this behavior, it's unwise to give a function that takes
+      a single composite-type argument the same name as any of the fields of
+      that composite type.
+     </para>
     </tip>
 
     <para>