Repro steps

1. Create two F# projects, LibProject and TestProject, where TestProject references LibProject
2. In LibProject, write the following code:

   module internal LibProject.Tools
   
   open System.Net
   open System.Runtime.CompilerServices
   
   [<assembly: InternalsVisibleTo("TestProject")>]
   do ()
   
   let inline int32FromBigEndian(num : int) =
       IPAddress.NetworkToHostOrder(num)

3. In TestProject, write the following code:

   module TestProject.Tests
   
   open LibProject.Tools
   
   let testTheFunction() =
       let xx = int32FromBigEndian 100
       0

The repro solution is attached: ReproTestProject.zip

Expected behavior

I think that the code should compile successfully. But even if it isn't, then the compiler should print a sensible error message.

Actual behavior

The code doesn't compile and the compiler provides rather cryptic messages:

LibProject\LibProject.fs(9,12): Error FS1113 : The value 'int32FromBigEndian' was marked inline but its implementation makes use of an internal or private function which is not sufficiently accessible
TestProject\TestProject.fs(6,14): Warning FS1116 : A value marked as 'inline' has an unexpected value
TestProject\TestProject.fs(6,14): Error FS1118 : Failed to inline the value 'int32FromBigEndian' marked 'inline', perhaps because a recursive value was marked 'inline'

Known workarounds

Well, don't use inline functions within internal modules with the InternalsVisibleTo attribute :)

Related information

Reproduced using .NET Core SDK:

$ dotnet --info
.NET Core SDK (reflecting any global.json):
 Version:   2.2.300
 Commit:    73efd5bd87

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.18362
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\2.2.300\

Host (useful for support):
  Version: 2.2.5
  Commit:  0a3c9209c0