[release/dev17.8] Fix - 16118 : Regression: Cannot use two or more open sessions in FCS…

src/Compiler/Checking/CheckDeclarations.fs

@@ -260,7 +260,7 @@ let BuildRootModuleContents (isModule: bool) enclosingNamespacePath (cpath: Comp
         ||> List.foldBack (fun id (cpath, moduleContents) -> (cpath.ParentCompPath, wrapModuleOrNamespaceContentsInNamespace isModule id cpath.ParentCompPath moduleContents))
         |> snd
 
-/// Try to take the "FSINNN" prefix off a namespace path
+/// Try to take the "FSI_NNN" prefix off a namespace path
 let TryStripPrefixPath (g: TcGlobals) (enclosingNamespacePath: Ident list) = 
     match enclosingNamespacePath with 
     | p :: rest when

src/Compiler/Interactive/fsi.fs

@@ -1837,7 +1837,7 @@ type internal FsiDynamicCompiler
         let opts =
             {
                 ilg = tcGlobals.ilg
-                outfile = multiAssemblyName + ".dll"
+                outfile = $"{multiAssemblyName}-{dynamicAssemblyId}.dll"
                 pdbfile = Some(Path.Combine(scriptingSymbolsPath.Value, $"{multiAssemblyName}-{dynamicAssemblyId}.pdb"))
                 emitTailcalls = tcConfig.emitTailcalls
                 deterministic = tcConfig.deterministic
@@ -3306,8 +3306,8 @@ type internal MagicAssemblyResolution() =
                 | None ->
                     // Check dynamic assemblies by simple name
                     match fsiDynamicCompiler.FindDynamicAssembly(simpleAssemName, false) with
-                    | Some asm -> asm
-                    | None ->
+                    | Some asm when not (tcConfigB.fsiMultiAssemblyEmit) -> asm
+                    | _ ->
 
                         // Otherwise continue
                         let assemblyReferenceTextDll = (simpleAssemName + ".dll")

tests/FSharp.Compiler.ComponentTests/Scripting/Interactive.fs

@@ -5,8 +5,11 @@ namespace Scripting
 open Xunit
 open System
 open FSharp.Test.Compiler
+open FSharp.Compiler.Interactive.Shell
+open FSharp.Test.ScriptHelpers
 
 module ``Interactive tests`` =
+
     [<Fact>]
     let ``Eval object value``() =
         Fsx "1+1"
@@ -31,6 +34,35 @@ module ``Interactive tests`` =
             (Warning 2304, Line 1, Col 3, Line 1, Col 13, "Functions with [<EntryPoint>] are not invoked in FSI. 'myFunc' was not invoked. Execute 'myFunc <args>' in order to invoke 'myFunc' with the appropriate string array of command line arguments.")
         ]
 
+    [<Theory>]
+    [<InlineData(true)>]
+    [<InlineData(false)>]
+    let ``Evaluation of multiple sessions should succeed`` (useMultiEmit) =
+
+        let args : string array = [| if useMultiEmit then "--multiemit+" else "--multiemit-"|]
+        use sessionOne = new FSharpScript(additionalArgs=args)
+        use sessionTwo = new FSharpScript(additionalArgs=args)
+
+        sessionOne.Eval("""
+module Test1 =
+
+    let test1 obj = sprintf "Execute - Test1.test1 - %A" obj""") |> ignore
+
+        let result1 = sessionOne.Eval("""Test1.test1 18""") |> getValue
+        let value1 = result1.Value
+        Assert.Equal(typeof<string>, value1.ReflectionType)
+        Assert.Equal("Execute - Test1.test1 - 18", value1.ReflectionValue :?> string)
+
+        sessionTwo.Eval("""
+module Test2 =
+
+    let test2 obj = sprintf "Execute - Test2.test2 - %A" obj""") |> ignore
+
+        let result2 = sessionTwo.Eval("""Test2.test2 27""") |> getValue
+        let value2 = result2.Value
+        Assert.Equal(typeof<string>, value2.ReflectionType)
+        Assert.Equal("Execute - Test2.test2 - 27", value2.ReflectionValue :?> string)
+
 module ``External FSI tests`` =
     [<Fact>]
     let ``Eval object value``() =