Missing DU member with interface after v 4.20+

[OnurGumus]

Description

After upgrading to 4.20 or 4.21 one of some DU's don't generate members

Repro code

module Test

type IItem =
    abstract member Next: int
    abstract member IsSuccess: bool

type ParseSuccess = { Next: int }

type ParseResult =
    | Success of ParseSuccess

    interface IItem with
        member this.Next =
            match this with
            | Success ps -> ps.Next

        member this.IsSuccess =
            match this with
            | Success _ -> true

With 4.19 the following js is generated

import { Union, Record } from "./fable_modules/fable-library-js.4.19.0/Types.js";
import { union_type, record_type, int32_type } from "./fable_modules/fable-library-js.4.19.0/Reflection.js";

export class ParseSuccess extends Record {
    constructor(Next) {
        super();
        this.Next = (Next | 0);
    }
}

export function ParseSuccess_$reflection() {
    return record_type("Test.ParseSuccess", [], ParseSuccess, () => [["Next", int32_type]]);
}

export class ParseResult extends Union {
    constructor(Item) {
        super();
        this.tag = 0;
        this.fields = [Item];
    }
    cases() {
        return ["Success"];
    }
    get Next() {
        const this$ = this;
        return this$.fields[0].Next | 0;
    }
    get IsSuccess() {
        return true;
    }
}

export function ParseResult_$reflection() {
    return union_type("Test.ParseResult", [], ParseResult, () => [[["Item", ParseSuccess_$reflection()]]]);
}

Note the this.IsSuccess member exists here but not below.

with 4.20 and 4.21 we have

import { Union, Record } from "./fable_modules/fable-library-js.4.21.0/Types.js";
import { union_type, record_type, int32_type } from "./fable_modules/fable-library-js.4.21.0/Reflection.js";

export class ParseSuccess extends Record {
    constructor(Next) {
        super();
        this.Next = (Next | 0);
    }
}

export function ParseSuccess_$reflection() {
    return record_type("Test.ParseSuccess", [], ParseSuccess, () => [["Next", int32_type]]);
}

export class ParseResult extends Union {
    constructor(Item) {
        super();
        this.tag = 0;
        this.fields = [Item];
    }
    cases() {
        return ["Success"];
    }
    get Next() {
        const this$ = this;
        return this$.fields[0].Next | 0;
    }
}

export function ParseResult_$reflection() {
    return union_type("Test.ParseResult", [], ParseResult, () => [[["Item", ParseSuccess_$reflection()]]]);
}

Repl link:

https://fable.io/repl/#?code=FDAuE8AcFMAIEl6mgW1gXmLbsCGAjAZ1ACdcBjUWFVfaE2AOWgA8qAuWASwDtQscBYmUrVa9BIQDKAV3LlohQrE74A9moA2ICDFgAFXCULRZ8xcvSwA3gJw5mbTr372AvndiewUOIePQAEqKMppUmPYAPrBmCkqwagBmBkYmsRYg9i70iRRwiMhoAO5coAAWnvY0KHQM5VyEAHSO4ZX2OCi4oORlsPXKJeVt7bDR6fGQygC0AHywk82sriNiNRL9jfDScnGWw+2d3b39sIMVK+1jOxawAPqws30kMtDAQA&html=DwCwLgtgNgfAUHUBTAhgE3gAk8CSwqbhgAOAtEgI4CuAlgG4C8A5AMID2AdmEt2QCoBPEkmaYAxlx7cWPAB5gA9OGgBuCSBQAnAM75G1MADMyADmZYcO8VtokwmHVvGMARMRI6AXIsXVOJADWAOYAdJIQigACAO5IAEYRJFy8YDqK4tQ6YOwQFFBIeNw6UQCMoQBMoQAMGVk5eUgFRWmhELScoQBWOq4wwIrWtvaWwEN2Dk4u7mCk3r7+QWER0XGJucmcqek6mmiCaLlloeUAzIN7B7ltHd29-YM2E-ADIKgYCMDx7PsSUCg6HRuFAkciSbgoDpILR9ODYbC4QRkeKGHKcTD-eJNNzsdh9AYQJEo2ZcF6Kb77eCIZSQWBwIA&css=Q

Expected and actual results

Expected:

    get IsSuccess() {
        return true;
    }

Actual: not generated

• Operating system
• MacOS 15

[ncave]

@OnurGumus Look like this has to do with #3658, where union case testers are elided. In this case IsSuccessful is the same as the generated union case tester.

[OnurGumus]

I see, thanks for clarification. Well I got way by removing "Is" prefix, but that's an oddly breaking change for the unfamiliar. I eave it to you to close the issue or not.

[ncave]

@OnurGumus We need to distinguish between the interface member and the augmented union case tester.
Fixed in #3917