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Is it something that I'm doing wrong or is this a bug? I am attempting to declare an interface that exposes an Event property.

I have tried:

#light

type IEventSource =
    abstract Event : IEvent<string>

type EventSource() =
    let event = new Event<string>()
   
    interface IEventSource with
        member this.Event = event.Publish
   
// error FS0192: internal error: forceM: envGetMethB: failed: x = get_Event(...)
// Please build a small example that reproduces this problem and report it to fsbugs@microsoft.com.

It works fine when I don't declare and implement an interface:

type EventSource() =
    let event = new Event<string>()
   
    member this.Event = event.Publish

I should have clarified that I get the above when loading the code as a script in fsi.

If I try to compile that same code I get:
-----------------------------------------------
The local method 'Temp.EventSource'::'get_Event' was referenced but not declared
generic arity: 0
Error in mref_as_MethodDef_idx for mref = "get_Event", error: Microsoft.FSharp.Compiler.AbstractIL.BinaryWriter+Mdef_not_foundException: Exception of type 'Microsoft.FSharp.Compiler.AbstractIL.BinaryWriter+Mdef_not_foundException' was thrown.
   at Microsoft.FSharp.Compiler.AbstractIL.BinaryWriter.mdkey_as_MethodDef_idx(cenv cenv, mdkey mdkey)
   at Microsoft.FSharp.Compiler.AbstractIL.BinaryWriter.mref_as_MethodDef_idx(cenv cenv, MethodRef mref)
typ_as_MemberRef_parent: mspec should have been encoded as mdtMethodDef?
The local method 'Temp.EventSource'::'get_Event' was referenced but not declared
generic arity: 0
Error in mref_as_MethodDef_idx for mref = "get_Event", error: Microsoft.FSharp.Compiler.AbstractIL.BinaryWriter+Mdef_not_foundException: Exception of type 'Microsoft.FSharp.Compiler.AbstractIL.BinaryWriter+Mdef_not_foundException' was thrown.
   at Microsoft.FSharp.Compiler.AbstractIL.BinaryWriter.mdkey_as_MethodDef_idx(cenv cenv, mdkey mdkey)
   at Microsoft.FSharp.Compiler.AbstractIL.BinaryWriter.mref_as_MethodDef_idx(cenv cenv, MethodRef mref)
typ_as_MemberRef_parent: mspec should have been encoded as mdtMethodDef?

I'm using CTP 1.9.6.0 and it compiles, but I get the same output as you.

Thanks for trying that out, yashez. I'm filing a bug report right now. My initial attempt to file one failed which is why I posted here but after a second check I see that I mis-typed the address...sigh...