Optional rule content breaks completion

[AresEkb]

Langium version: 3.5.0
Package name: langium

Steps To Reproduce

Use the following grammar:

grammar Test

entry Document:
    Annotation
    'document' name=ID;

fragment Annotation:
    ('@annotation' annotation=STRING)?;

terminal ID: /[_a-zA-Z][\w_]*/;
terminal STRING: /'(\\.|[^'])*'/; 

hidden terminal WS: /\s+/;

Run completion for an empty document

The current behavior

It will show only one proposal: @annotation

The expected behavior

But two proposals are expected: @annotation and document

Additional information

If you replace fragment by the embedded rule, then completion works as expected:

grammar Test

entry Document:
    ('@annotation' annotation=STRING)?
    'document' name=ID;

terminal ID: /[_a-zA-Z][\w_]*/;
terminal STRING: /'(\\.|[^'])*'/; 

hidden terminal WS: /\s+/;

[msujew]

It doesn't seem to be related to the fragment rule specifically, this grammar exhibits the same behavior:

grammar Test

entry Document:
    annotation=Annotation
    'document' name=ID;

Annotation:
    ('@annotation' annotation=STRING)?;

However, this works correctly:

grammar Test

entry Document:
    annotation=Annotation? // <-- moving the optionality here
    'document' name=ID;

Annotation:
    ('@annotation' annotation=STRING);

Therefore this also seems to work as a workaround for this issue (moving the optionality to the caller side of the fragment rule):

grammar Test

entry Document:
    Annotation?
    'document' name=ID;

fragment Annotation:
    '@annotation' annotation=STRING;

terminal ID: /[_a-zA-Z][\w_]*/;
terminal STRING: /'(\\.|[^'])*'/; 

hidden terminal WS: /\s+/;

I don't think we're dealing with the case correctly where the whole rule content is optional. This is a rather rare case (which is probably why no one noticed this yet) but makes sense to have on fragment rules.

[AresEkb]

Thanks for the workaround! It works for me