`AmbiguousMatchException` on `System.Text.Json` deserialization with `FSharp.Core` version `9.0.300`

[WithinUA]

When I deserialize FSharp class I get this exception

Repro steps

just download and run this project

MultipleCompilationMappingAttributesRepro.zip

Expected behavior

No exception

Actual behavior

System.Reflection.AmbiguousMatchException: Multiple custom attributes of the same type 'Microsoft.FSharp.Core.CompilationMappingAttribute' found.

Known workarounds

Downgrade to 9.0.202 or lower

Related information

• .NET 8, 9

[T-Gro]

This was addressed with dotnet/runtime#114961 , duplicate of dotnet/runtime#114824

[cmeeren]

As far as I can tell, a fix was published in .NET 9.0.6 (SDK 9.0.301). At least that was the milestone used here. But when running with FSharp.Core 9.0.300 on .NET 9.0.6, I still get this error. It therefore seems I have no recourse but to downgrade FSharp.Core to 9.0.202.

When can I safely upgrade to FSharp.Core 9.0.300?

[T-Gro]

@cmeeren : This did not meet the dotnet/runtime's bar for a servicing, and the change only applies to .NET 10, including preview versions of .NET 10.
The combination of .NET 9 runtime and 9.0.300 FsharpCore will still have this error unfortunately.