Allowed to return possibly TOASTed datums?

Hi,

I would like to implement a function that simply returns its argument
(for the unary plus operator of a custum data type).

Is it allowed to use the following function, assuming the data type is
TOASTable?

PG_FUNCTION_INFO_V1(mytype_pos);
Datum mytype_pos(PG_FUNCTION_ARGS) {
PG_RETURN_DATUM(GETARG_DATUM(0));
}

CREATE FUNCTION "mytype_pos"("mytype") RETURNS "mytype"
STRICT IMMUTABLE LANGUAGE c AS 'mylib.so', 'mytype_pos';

CREATE OPERATOR + (
rightarg = "mytype",
function = "mytype_pos"
);

Or do I need to always return a DETOASTed value? In addition to the
answer, I would like to know where I can find such information. It
seems to be difficult (for me) to find it in the source. Is there any
other documentation I can use, or any hints on WHERE in the source I
find more information about when to DETOAST and when not?

Also: Is my approach an idiomatic way to implement unary plus? Are there
pre-existing functions for that?

Many thanks in advance for your help and kind regards

Jan Behrens