Missing implementation for enum extension value

[achim-t]

Related to AL0596

For Enums, that implement an interface, all values must provide an implementation. Unless the enum provides a DefaultImplementation.
The proposed rule would require an implementation even when there is a DefaultImplementation.

Reasoning
We extended enum 5057 "Contact Business Relation Link To Table" without looking too closely. The DefaultImplementation throws a runtime error.

codeunit 5557 "Contact BRL Default" implements "Contact Business Relation Link"
{
    var
        Msg: Label 'No implementation provided for Contact Relation Link', Locked = true;

    procedure GetTableAndSystemId(No: Code[20]; var TableId: Integer; var SystemId: Guid): Boolean
    begin
        TableId := 0;
        SystemId := CreateGuid();
        Error(Msg);
    end;
}

[Arthurvdv]

Thanks for raising this — the pain is real (we got bitten by exactly this Contact BRL Default runtime error), but after a critical review I don't think the rule works as proposed. Here's the breakdown.

The core problem with a blanket rule

Requiring an explicit Implementation even when a DefaultImplementation exists contradicts the documented purpose of the property. The Microsoft docs are explicit: "Providing a default implementation by using the DefaultImplementation property can be used to make such new additions non-breaking."

And the data backs that up. Scanning the current BC source: 33 distinct DefaultImplementation usages. Only about a quarter are error-throwing or no-op placeholders (Contact BRL Default, Shpfy OpenDoc NotSupported, E-Doc. Unspecified Impl., Voucher No Check). The other ~75% are genuine working defaults (Basic SMTP Auth, Transform. Rule - Custom, Job Queue Start Report Runner, ...). A blanket rule would fire falsely on the majority and train people to suppress it.

The "smart" variant doesn't work either

The obvious refinement — only flag when the default implementation throws — is not implementable in practice. Symbol packages (SymbolReference.json) contain signatures, not method bodies. When the enum comes from a dependency (the 99% case: BaseApp as symbols), the analyzer cannot see that Contact BRL Default calls Error(). Name heuristics are too fragile: Contact BRL Default throws, Default File Scenario Impl. works fine.

How other linters handle this

This lint category exists elsewhere, and it's always opt-in:

• Rust Clippy missing_trait_methods is the exact analogue (implement all trait methods even when defaults exist). It's in the restriction group, allow-by-default, because relying on defaults is usually intentional.
• C#/Roslyn: no analyzer forces overriding C# 8 default interface methods.
• Java (ErrorProne, PMD, Checkstyle): nothing forces overriding default methods.

What could actually work

A narrower, declaration-side rule: flag an enum you declare whose DefaultImplementation codeunit unconditionally throws — "an always-throwing default defeats the non-breaking purpose; provide a working default or omit the property so AL0596 forces extenders to implement." Source is available on the declaration side, so this is fully analyzable, and it stops the anti-pattern from spreading into partner code.

This rule would ship disabled by default, to be enabled through a custom ruleset. To be fully honest: I'm unsure how many people would turn it on. It can't catch the case that actually bit us (BaseApp ships as symbols, so the throwing default is invisible to the analyzer), it only prevents you from creating the same trap yourself. That's a fairly narrow audience.

The real fix is upstream

Arguably this is a platform gap: AL has no way to mark a DefaultImplementation as "placeholder, please implement" (the way Obsolete communicates intent). That would give the compiler itself the information no analyzer can recover from symbols. Worth raising on microsoft/AL in parallel.

That said, I'd genuinely like to make something out of this — do you have ideas to improve the rule so it could catch the dependency-side case, or another angle we've both missed?

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