Check if type exists when tracing type cast, is, and introspect.

[dnwpark]

Given the schema:

  type MyType {
    parent: MyType;
    is_initial := .parent is null;
  }

A nicer error is produced:

error: type 'default::null' does not exist
  │
6 │     is_initial := .parent is null;
  │                              ^^^^ Did you mean to use `exists`?

close #7256

[msullivan]

We might want some more details in the message? Or maybe just use the span of the whole IS when it is none or null?

[dnwpark]

@msullivan After thinking a bit, I think that changing the span could be confusing for particularly long expressions. What do you think of changing the hint to Did you mean to use exists to check if a set is empty?

Another thing to consider is that the right side of is can also be a type expression (eg. is_initial := .parent is MyType | null;). I think it's ok to not add the hint here.

[msullivan]

That sounds reasonable

[dnwpark]

@msullivan take a look when you can