fix(inline, expand): Only accept on structures, assocs or table aliases

[patricebender]

an expand or inline on a many type has no defined behavior, yet.
→ as of now this is an error because the path cant be resolved (expand / inline without .elements)

we should reject it with a proper error, just as the compiler does for static views.

[johannes-vogel]

afaik, collection valued properties are not queried with expand but as a regular column

[patricebender]

Okay, this PR aligns the runtime with the compiler behavior. I would say, this change is fine as-is because it definetly improves the current error situation. If we come up with some meaningful semantics I'd be happy to implement it 👍

[johannes-vogel]

the error msg however is not clear to me.

"Unexpected “expand” on “emails”; can only be used after a reference to a structure, association or table alias does not tell me that I should use it as a regular column instead of expanding it.

[patricebender]

How about Expecting a reference to a structure, association, or table alias before using '{ ‹expand› }'.

→ The error message which comes with this change tries to tell the user that the expand on the arrayed element is unexpected. In the second part of the message, I try to give guidance on which columns the expand could be used.

In the compiler the error message is very similar:

Error[def-unexpected-nested-proj]: Unexpected ‘{ ‹expand› }’; can only be used after a reference to a structure, association or table alias
    |
  e.cds:23.16-25.6, at entity:“A”/column:“manyStruct”
    |
 23 |     manyStruct {
 24 |       leaf
 25 |     }

---

Navigating into an arrayed structure is not possible, yet. At least its not supported by the compiler SQL Backend.

entity Authors {
  key ID         : Int16;
      name       : String(111);
      manyStruct : many {
        leaf   : Integer;
        sub    : {
          leaf : Integer;
        }
      };
}

will in the end only be:

-- generated by cds-compiler version 4.8.1
CREATE TABLE Authors (
  ID SMALLINT NOT NULL,
  name NVARCHAR(111),
  manyStruct NCLOB, -- leafs are all gone
  PRIMARY KEY(ID)
);

if you try to navigate into the struct:

Error[ref-unexpected-many-navigation]: Unexpected navigation into arrayed structure
    |
  e.cds:23:5, at entity:“A”/select/column:“leaf”
    |
 23 |     manyStruct.leaf
    |     ^

[patricebender]

@stewsk please have a look