[Bug]: Can't apply @minValue decorator on an named union member

[artsiommiksiuk]

Describe the bug

I can do this:

model A {
  @minValue(0)
  index: int32 | "auto";
}

but can't apply @minValue(0) decorator to the same union defined in a form of a named union.

Reproduction

union U {
  @minValue(0)
  number: int32;
  "auto";
}

model B {
  index: U
}

Decorator throws an error:

Cannot apply @minValue decorator to int32 since it is not assignable to numeric | utcDateTime | offsetDateTime | plainDate | plainTime | duration | ModelProperty

https://typespec.io/playground/?e=%40typespec%2Fopenapi3&c=bW9kZWwgQSB7CiAgQG1pblZhbHVlKDApCiAgaW5kZXg6IGludDMyIHwgImF1dG8iOwp9CgoKdW5pb24gVdQ2bnVtYmVyxzc7CiDMOMZtQsU3x15VCn0%3D&options=%7B%7D&vs=%7B%7D

Checklist

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• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.
• The provided reproduction is a minimal reproducible example of the bug.

[witemple-msft]

@artsiommiksiuk That error message is definitely misleading. @minValue cannot be applied to union variants, but the message is telling you the wrong thing by printing just int32. I'll definitely fix the error message and look into whether we can allow @minValue and @maxValue on union variants (but I think the answer will be that we can't do that safely, since emitters typically treat union variants as equivalent to their types).

There is a workaround, though. You can declare a custom scalar that has @minValue:

@minValue(0)
scalar NonZeroInt32 extends int32;

union U {
  number: NonZeroInt32;
  "auto";
}

model B {
  index: U;
}

However, I will also suggest that uint32 could be a better option instead of int32 unless you really need a signed integer type.

[artsiommiksiuk]

@witemple-msft Thanks for checking.

I think this must be supported. As per typespec documentation: emitters are responsible for mapping and selecting what they can output. E.g. openapi, as well as json schema are supporting this combination. Otherwise decorators are being emitter dependent and restricted, instead of being open and selective. With the workaround you've mentioned, the only difference I see as a user is explicit scalar declaration vs implicit constraining of a type. Like:

@minValue(0)
scalar A extends int32;

and

@minValue(0)
index: int32;

should produce the same constraints. Explicit scalar would result in just having a name, therefore, using scalar or decorated base type should be the equivalent. IMO.

But, there might be another explanation. I've already shown with not named union example that you can apply decorator, but I did also non equivalent example with named union (applied @minValue to single variant, instead of whole union). If I apply @minValue(0) to the whole union U, then it is working the same, and which might be actually a real confusion from my side. I wanted only real index to be min valued, not whole union.

But the message, is, yes, misleading and, I still think, decorating union member should be supported the same way as if it was a model's property.