Fix referenceassemblygeneration for .exe files (#13963)

* Fix referenceassemblygeneration for .exe files

* Tests

Author: KevinRansom

src/Compiler/AbstractIL/ilwrite.fs

@@ -1154,7 +1154,7 @@ let canGenMethodDef (tdef: ILTypeDef) cenv (mdef: ILMethodDef) =
         | ILMemberAccess.Public -> true
         // When emitting a reference assembly, do not emit methods that are private/protected/internal unless they are virtual/abstract or provide an explicit interface implementation.
         | ILMemberAccess.Private | ILMemberAccess.Family | ILMemberAccess.Assembly | ILMemberAccess.FamilyOrAssembly
-            when mdef.IsVirtual || mdef.IsAbstract || mdef.IsNewSlot || mdef.IsFinal -> true
+            when mdef.IsVirtual || mdef.IsAbstract || mdef.IsNewSlot || mdef.IsFinal || mdef.IsEntryPoint -> true
         // When emitting a reference assembly, only generate internal methods if the assembly contains a System.Runtime.CompilerServices.InternalsVisibleToAttribute.
         | ILMemberAccess.FamilyOrAssembly | ILMemberAccess.Assembly
             when cenv.hasInternalsVisibleToAttrib -> true

tests/fsharp/Compiler/CodeGen/EmittedIL/ReferenceAssemblyTests.fs

@@ -982,4 +982,35 @@ extends [runtime]System.Object
   } 
 
 } """ ]
+
+    [<Test>]
+    let ``Build .exe with --refonly ensure it produces a main in the ref assembly`` () =
+        FSharp """module ReferenceAssembly
+open System
+
+Console.WriteLine("Hello World!")"""
+        |> withOptions ["--refonly"]
+        |> withName "HasMainCheck"
+        |> asExe
+        |> compile
+        |> shouldSucceed
+        |> verifyIL [
+            referenceAssemblyAttributeExpectedIL
+            """.class private abstract auto ansi sealed '<StartupCode$HasMainCheck>'.$ReferenceAssembly
+       extends [mscorlib]System.Object
+{
+  .method public static void  main@() cil managed
+  {
+    .entrypoint
+    // Code size       2 (0x2)
+    .maxstack  8
+    IL_0000:  ldnull
+    IL_0001:  throw
+  } // end of method $ReferenceAssembly::main@
+
+} // end of class '<StartupCode$HasMainCheck>'.$ReferenceAssembly
+"""
+        ]
+        |> ignore
+
     // TODO: Add tests for internal functions, types, interfaces, abstract types (with and without IVTs), (private, internal, public) fields, properties (+ different visibility for getters and setters), events.