Incorrect types inferrence for parents with own properties

[pluralia]

When a parent has no own properties, we generate a type Parent :

Parent:
    ChildA | ChildB;

ChildA:
    'a' a=NUMBER commonProp=ID;

ChildB:
    'b' b=NUMBER commonProp=ID;

generates

export type Parent = ChildA | ChildB;

export interface ChildA extends AstNode {
    a: number
    commonProp: string
}

export interface ChildB extends AstNode {
    b: number
    commonProp: string
}

Besides, common children properties like commonProp will be lifted automatically. However, as only a parent has own properties, we generate an interface Parent:

Parent:
    parentProp=ID (ChildA | ChildB);

// ChildA and ChildB are without changes

generates

export interface Parent extends AstNode {
    parentProp: string
}

export interface ChildA extends AstNode {
    a: number
    commonProp: string
}

export interface ChildB extends AstNode {
    b: number
    commonProp: string
}

You can notice, that ChildA and ChildB don't extend Parent and their commonProp is not lifted in Parent.

Langium version: 0.5.0 Package name: langium

The current behavior

Inferred Parent -- ChildA -- ChildB hierarchy is wrong when Parent has own properties (look above).

The expected behavior

export interface Parent extends AstNode {
    parentProp: string
    commonProp: string
}

export interface ChildA extends Parent {
    a: number
}

export interface ChildB extends Parent {
    b: number
}

The solution to the problem requires restoring properties lifting algorithm.

[sailingKieler]

However, as only a parent has own properties, we generate an interface Parent:

Parent:
    parentProp=ID (ChildA | ChildB);

// ChildA and ChildB are without changes

generates

export interface Parent extends AstNode {
    parentProp: string
}

export interface ChildA extends AstNode {
    a: number
    commonProp: string
}

export interface ChildB extends AstNode {
    b: number
    commonProp: string
}

You can notice, that ChildA and ChildB don't extend Parent and their commonProp is not lifted in Parent.

Langium version: 0.5.0 Package name: langium

@pluralia IMO the behavior is totally correct. Actually you grammar is incorrect, as the rule

Parent:
    parentProp=ID (ChildA | ChildB);

needs to instantiate an object of type Parent once it consumes an ID in order to assign it to prop parentProp. The result of ChildA | ChildB is then unassigned (and eventually lost).

If you can write your grammar like

Parent:
    (ChildA | ChildB) parentProp=ID;

you should obtain your desired result.

Alternatively you could use assigned actions, like

Parent:
    parentProp=ID (ChildA.props = current | ChildB.props = current);

(in order to keep your syntax, of course the AST will still look different).

The current behavior

Inferred Parent -- ChildA -- ChildB hierarchy is wrong when Parent has own properties (look above).

The expected behavior

export interface Parent extends AstNode {
    parentProp: string
    commonProp: string
}

export interface ChildA extends Parent {
    a: number
}

export interface ChildB extends Parent {
    b: number
}

[pluralia]

If you can write your grammar like

Parent:
    (ChildA | ChildB) parentProp=ID;

you should obtain your desired result.

@sailingKieler, could you explain, why this has to get different result with?

Parent:
    (ChildA | ChildB) parentProp=ID;

[sailingKieler]

Parent:
    (ChildA | ChildB) parentProp=ID;

In this case the rule doesn't instantiate an object itself, but proceeds with the result of (ChildA | ChildB). It's the same pattern as for rules like:

Expression:
   Literal | UnaryOp | BinaryOp
;

plus the additional property assignment

[pluralia]

Thank you for the explanation!

If you can write your grammar like

Parent:
    (ChildA | ChildB) parentProp=ID;

you should obtain your desired result.

This infers the same types and the issue is still here.

[sailingKieler]

This infers the same types and the issue is still here.

Ok, then it's really a mistake ;-). Does Xtext's type inference test contain a corresponding test case?

[spoenemann]

Yes, in Xtext it's forbidden to call unassigned rules after object creation (action or property assignment). But in Langium we are not forced to do the same, see discussion in https://github.com/langium/langium/discussions/205.