Self CAST has to be invariant,
that is the operation done on an expression that the value converts in it
same type has to return the same value of departure;
this is the first rule of a function of conversion type,
and this is seen by the diagonal of the chart of documentation,
the diagonal has "F" in all the values.

Well, since CAST refers to an expression and not to a Field,
they don't have to be us the tied up restrictions to the fields of a chart.
When CAST is used with a "C,V,Q" VFP imposes
how limit to the length nFieldWidth the maximum length of a field.
The limit that has to be imposed is 2^31-1, not 2^24-32 (engine max buffer size),
because it is possible to load a variable with more than 2^24-32 bytes.

Repro:

CLEAR

? InvariantCast(SPACE(254))
? InvariantCast(SPACE(255))  && this fire a error

? InvariantCast(0h+SPACE(254))
? InvariantCast(0h+SPACE(255))	&& this fire a error

PROCEDURE InvariantCast(x)
return x==CAST(X AS (VARTYPE(X))(LEN(X)))

Then:

X=SPACE(3000)
? VARTYPE(CAST(m.x AS W))

* but it is impossible to impose a greater length of 254
? CAST(m.x AS Q(300))

* Of course, There are many workaroundes , but none can overcome 2^24-32

? LEN(CAST(LEFT(m.x,300) AS W))

* more 2^24-32 bytes conversion
? LEN(CAST(FILETOSTR(GETFILE("","","",0,"Load a file with more than 16Mbytes")) as w))