Re: Proposal for resolving casting issues

I wrote:
> I think we must extend pg_cast's castimplicit column to a three-way value:
>     * okay as implicit cast in expression (or in assignment)
>     * okay as implicit cast in assignment only
>     * okay only as explicit cast

> Question: what shall we call these alternatives in CREATE CAST?  The SQL99
> phrase AS ASSIGNMENT looks like it should mean the second, but I think
> the spec semantics require it to mean the first.  Ugh.  Perhaps AS
> ASSIGNMENT ONLY for the second case?

On looking more closely, SQL99 appears to define user-defined casts as
invocable *only* in explicit cast and assignment contexts.  Part 2 sez:
        4.13  Data conversions
        Explicit data conversions can be specified by a CAST operator.        A CAST operator defines how values of a
sourcedata type are        converted into a value of a target data type according to        the Syntax Rules and
GeneralRules of Subclause 6.22, "<cast        specification>". Data conversions between predefined data types
andbetween constructed types are defined by the rules of this part        of ISO/IEC 9075. Data conversions between one
ormore user-defined        types are defined by a user-defined cast.
 
        A user-defined cast identifies an SQL-invoked function, called the        cast function, that has one SQL
parameterwhose declared type is        the same as the source data type and a result data type that is the
targetdata type. A cast function may optionally be specified to        be implicitly invoked whenever values are
assignedto targets of        its result data type. Such a cast function is called an implicitly        invocable cast
function.

This seems to mean that we can get away with defining AS ASSIGNMENT to
mean my second category (implicit in assignment only), and then picking
some more natural term for my first category (implicit anywhere).

I favor using IMPLICIT, which would make the syntax of CREATE CAST be
   CREATE CAST (sourcetype AS targettype)       WITH FUNCTION funcname (argtype)       [ AS ASSIGNMENT | IMPLICIT ]
CREATE CAST (sourcetype AS targettype)       WITHOUT FUNCTION       [ AS ASSIGNMENT | IMPLICIT ]
 

Or possibly it should be AS IMPLICIT?

Comments?
        regards, tom lane