Interesting issue, noticed a few things:

• The original schema doesn't work but does if you constrain on just @date (and reproduces the error)
• I think this is an edgedb and not edgedb-cli issue because the CLI just uses create migration to (schema) and then describe current migration as json which gives it this prompt from the server which it simply follows:

  "parent": "m1t2it77gg4vsiznipiim453v3b7z6hhjy3v6j7e2xumflsmwjtc7q",
  "complete": false,
  "proposed": {
    "prompt": "did you alter the type of property 'date' of link 'habit_completed'?",
    "data_safe": false,
    "prompt_id": "SetPropertyType PROPERTY default::__|date@default|__||habit_completed&default||User",
    "confidence": 1.0,
    "statements": [
      {
        "text": "ALTER TYPE default::User {\n    ALTER LINK habit_completed {\n        ALTER PROPERTY date {\n            SET TYPE cal::local_date USING (\\(cast_expr));\n        };\n    };\n};"
      }
    ],
    "required_user_input": [
      {
        "prompt": "Please specify a conversion expression to alter the type of property 'date'",
        "new_type": "cal::local_date",
        "old_type": "std::datetime",
        "placeholder": "cast_expr",
        "pointer_name": "date",
        "new_type_is_object": false,
        "old_type_is_object": false
      }
    ]
  },
  "confirmed": []
}

(Correction: looks like this is indeed a CLI issue: per @msullivan the server just reports the old and new type) https://github.com/edgedb/edgedb-cli/blob/master/src/migrations/create.rs#L332

The suggested cast is produced by the CLI, not by the compiler/server. The server just reports the old_type and the new_type back to the CLI

• Casting into a cal::local_date won't work in any case whereas something like this will: cal::to_local_date(@date, 'UTC') So maybe the prompt should be cal::to_local_date(@date, <timezone>) or something like that? Since the local_date will vary depending on which timezone is being used as the local time.

You're correct that the original cast suggestion won't work, even if it references the property correctly, but I wasn't sure how sophisticated and bespoke those suggestions could be. Yeah, this suggestion you made (cal::to_local_date(@date, <timezone>)) is basically what I ended up using to do the cast.

So, maybe this is actually two issues:

1. reference link properties properly when giving a cast expression suggestion and
2. give a working suggestion when changing a datetime property to a cal::local_date

You're correct that the original cast suggestion won't work, even if it references the property correctly, but I wasn't sure how sophisticated and bespoke those suggestions could be. Yeah, this suggestion you made (cal::to_local_date(@date, <timezone>)) is basically what I ended up using to do the cast.

So, maybe this is actually two issues:

1. reference link properties properly when giving a cast expression suggestion and
2. give a working suggestion when changing a datetime property to a cal::local_date

I was looking into this today and looks like it's pretty easy to just query the database with select schema::Cast and select schema::Function and then put together a HashMap to tell which casts are doable, and if not castable then which functions could work. So I think I can make this more intuitive in addition to the link property fix.