Example:

model Bar {
  name: utcDateTime;
}

@@Tcgc.alternateType(Bar.name, string, "python");

Rationale is to be able to express in TypeScript autorest Swagger directive rules like this one:

- from: swagger-document
  where: $.definitions.AccessPolicy.properties
  transform: >
    $.Start.format = "str";
    $.Expiry.format = "str";

Side note, if I understood @timotheeguerin correctly, we should be able to make this work as well:

scalar storageDateTime extends utcDataTime;
model Bar {
  name: storageDateTime;
}

@@Tcgc.alternateType(storageDateTime, string, "python");

which would avoid to replace all one by one.

I would expect this is done in a way that emitter do not need to change anything, since they will receive models with the new type auto-magically.