Issue in case of parsing recovery on cross reference ?

[ydaveluy]

Hi,

Sometimes when typing in my dsl I have error about calling ref on an undefined object. It appears for example when the ID is not written in the grammar and is deduced by parsing recovery.
All my code need to check if the attribute is defined before accesing the ref: element.attribute?.ref.
In the element interface the attribute is not optional.

This behaviour can be seen in the playground: https://langium.org/playground?grammar=OYJwhgthYgBAEgUwDbIPYHU0mQEwFD6IB2ALiAJ6wCyauKAXPrC7ABQAOiIAzmsTwDUAXgAK3PsVgAfWKESJSAS2LAhwgOIgFy1QEoAVAG5C43vyatYAci7ni12MUiJhASQAiJ-Fp0rglqzWSKhojnaSwgDaZpIMngC6NgCE1t4AFkq49FKk3BAqYMiwGADKDLAA9AA6QpUmeSAFzsWeFZVRAPpgALQAXgCCPQBaCVHVAO6dCQb1hJnZJLCNzUU0ADKdAMIA8tTUAKIAcgAq7dU1BuM81aUzAPzVBhdzCznL%2BYXFpZu7%2B8dnKoXC5RAB61WI1RAMzmQA&content=A4UwTgzg9gdgBAKSgCxgKFJWiCGMRpoASIANqVIijAITFkVxpA
Generated AST:

    {
        $type: 
            "Model", 
        persons: 
            [
                {
                    $type: 
                        "Person", 
                    name: 
                        "John"
                }, 
                {
                    $type: 
                        "Person", 
                    name: 
                        "Jane"
                }
            ], 
        greetings: 
            [
                {
                    $type: 
                        "Greeting", 
                    person: 
                        Reference('#/persons@0')
                }, 
                {
                    $type: 
                        "Greeting"
                }
            ]
    }

In the last Greeting the person reference is not defined.

[ydaveluy]

In the parser the tokens inserted in recovery are not considered as ValidToken:

langium/packages/langium/src/parser/langium-parser.ts

Line 321 in c3afb24

return !token.isInsertedInRecovery && !isNaN(token.startOffset) && typeof token.endOffset === 'number' && !isNaN(token.endOffset);

when the ast is built the token is not processed:

langium/packages/langium/src/parser/langium-parser.ts

Line 295 in c3afb24

if (!this.isRecording() && this.isValidToken(token)) {

Maybe a different logic could in case of a recovery token build the ast with a default value:

• empty string for mandory strings
• NaN for mandatory numbers
• Reference for a cross-reference
• ...