FS1099 when instantiating internal delegate from another assembly

[fsoikin]

When using a lambda expression to instantiate an internal delegate defined in another assembly (available via InternalsVisibleTo), the compiler issues FS1099: No Invoke methods found for delegate type.

Repro steps

a.cs:

[assembly: System.Runtime.CompilerServices.InternalsVisibleTo("b")]
namespace Repro {
 internal delegate string D( int x );
 internal class C { public C( D d ) {} }
}

b.fs:

module ReproFS
let c = new Repro.C( fun x -> "" )

compile:

> csc a.cs /out:a.dll /target:library
> fsc b.fs --out:b.dll --target:library -r:a.dll

Expected behavior

Successful compilation.

Actual behavior

b.fs(2,20): error FS1099: No Invoke methods found for delegate type

Known workarounds

1. Specifying the delegate type explicitly works: new Repro.C( Repro.D(fun x -> "") )
2. If the delegate is itself defined in F# (i.e. type D = delegate of int -> string), the error is different: FS0002: This function takes too many arguments, or is used in a context where a function is not expected (though this doesn't have to do with it being internal).
3. And of course, making the delegate public removes the issue.

Related information

• Operating system: Windows
• Branch: Compiler installed, not built locally. Microsoft (R) F# Compiler version 14.0.23413.0
• .NET Runtime, CoreCLR or Mono Version: .NET Framework 4.6