CS9058 compiler (but not doc) message can be misleading

[RexJaeschke]

Type of issue

Missing information

Description

The following code compiles under V12:

class C;
struct S;
interface I;
enum E;

which allows bodies of these declarations to be omitted.

However, when compiled with earlier versions, each results in CS9058.

Now the on-line docs for this error say

CS9058: Feature is not available in C# xx. Please use language version 12 or greater.

which is correct. However, the message reported by the compiler is, as follows:

[CS9058] Feature 'Primary constructors' is not available in language C# xx. Please use language version 12 or greater.

This is misleading, as there are no primary constructors being used here. It's just that the MS spec for the primary constructor feature smuggled in the ability to omit these bodies. So, a better compiler message would be helpful.

Page URL

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-messages/feature-version-errors?f1url=%3FappId%3Droslyn%26k%3Dk(CS9058)

Content source URL

https://github.com/dotnet/docs/blob/main/docs/csharp/language-reference/compiler-messages/feature-version-errors.md

Document Version Independent Id

b75d77df-8629-28f8-bc46-b8bc1a4b8ab1

Platform Id

17463722-03e9-8158-01e2-2da0991a1564

Article author

@BillWagner

Metadata

• ID: 46e75de6-8d41-b986-5f95-67a1149c6d8c
• PlatformId: 17463722-03e9-8158-01e2-2da0991a1564
• Service: dotnet-csharp
• Sub-service: errors-warnings

Related Issues

[BillWagner]

Adding @jaredpar and @333fred to check my work.

It might be surprising, but it is correct in terms of how the parser works. In all these cases, it's considered a parameterless primary constructor.

@RexJaeschke If my analysis is correct, I think this should be taken into account for how we write the spec language for primary constructors.

[333fred]

Yes, this syntax was added as part of the primary constructors feature.

[RexJaeschke]

@BillWagner @333fred I'm not wed to having a change, but ...

First, there is no concept of a primary constructure for interfaces and enums.

Second, I thought the presence of the (...) is what triggered the generation of a primary constructor. If that is true, the class and struct in my example have no such parens. They certainly don't have primary constructors.

Bill, I'll chat with you offline re your thoughts of how this might affect the Ecma spec.

[333fred]

First, there is no concept of a primary constructure for interfaces and enums.

Sure, that's correct.

Second, I thought the presence of the (...) is what triggered the generation of a primary constructor.

That's also correct.

In terms of the message, the ; was added specifically in support of primary constructors. It wasn't "smuggled in", or anything like that. We added the syntax intentionally because we expected class Derived(int blah) : Base(blah); to be ok syntax. However, we don't feel the need to have a dedicated feature label, spec, and error message just for trailing semi-colons on types. It's a somewhat rare error, and not worth the effort in our opinion.