F# compiler does not respect ExperimentalAttribute

[nightroman]

The ExperimentalAttribute on a class in a C# library causes expected errors in C# consumer projects.
But this attribute seems to be ignored in F# consumer projects.

Repro steps

Please see the attached solution with 3 projects
2025-01-03-0427-ExperimentalAttribute.zip

1. MyLib, the library with Class1 with ExperimentalAttribute
2. MyAppCSharp, the C# project using Class1
3. MyAppFSharp, the F# project using Class1

Expected behavior

Compiling both MyAppCSharp and MyAppFSharp should cause the error

MY001 'MyLib.Class1' is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.

Actual behavior

MyAppFSharp compiles with no errors or warnings.
(C# project shows the error as expected)

Known workarounds

None.

Related information

• Windows 11
• .NET 9.0
• Visual Studio Community 2022 (64-bit) 17.12.3

[T-Gro]

The attribute https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-12.0/experimental-attribute is a C# feature of C# 12.
The interop with existing https://fsharp.github.io/fsharp-core-docs/reference/fsharp-core-experimentalattribute.html#:~:text=This%20attribute%20is%20used%20to%20tag%20values,that%20are%20part%20of%20an%20experimental%20library%20feature. does not exist.

The attribute's fields matches the proposal from better obsoletion (https://github.com/dotnet/designs/blob/main/accepted/2020/better-obsoletion/better-obsoletion.md#api-changes ), so I think this might be packed under the existing feature request of #8654

#8654 is a open feature request looking for an implementor.

[edgarfgp]

With the new properties DiagnosticId and FormatUrl the warning/errors message should account for this. Additionally we should consider converting the warning/error to use extended diagnostic feature so tooling can consume this and show in tooltips ?

[T-Gro]

Both of the properties should be exposed to tooling, yes .