Alternative including cross reference and string is recognized as reference to string

An alternative including a cross reference to X and strings sometimes can be recognized as a reference to X | string

A minimal example is as follows.

grammar Debug

entry Model: (items+=(A | B))*;

interface B {
    b: @A | string;
}

A infers A: 'A' a=ID;

B returns B: 'B' b=([A:ID] | 'C');

hidden terminal WS: /\s+/;
terminal ID: /[_a-zA-Z][\w_]*/;

Langium-vscode gives a diagnostic.

The assigned type 'Reference<'C' | A>' is not compatible with the declared property 'b' of type 'Reference<A> | string'.
''C' | A' is not expected.

The type of [A:ID]|'C' is recognized as Reference<'C'|A> instead of Reference<A>|'C'.

Adding an action to the rule B resolves this error: B returns B: {B} 'B' b=([A:ID] | 'C');.

If the rule B is written using infers instead of returns, there is no diagnostic, but in the generated ast, type of b is still incorrect. Also, adding an action in such case does not resolve the error.

[dhuebner]

@msujew Would give a nice error message: Mixing references and other types might break quite a few things in Langium. xD

[luan-xiaokun]

I like the error message 👍