Script compilations not specifying a return type take an implicit dependency on the host corlib

[abock]

When "cross compiling" a script for a corlib different from the "host", if a return type is not specified, System.Object from the host (via typeof(object)) is reflection-loaded.

This makes it impossible to target a script on a different corlib (e.g. Xamarin.iOS) from the host without explicitly specifying a returnType: via CreateScriptCompilation.

Example:

const string corlibPath = "/Library/Frameworks/Xamarin.Mac.framework/Versions/Current/lib/mono/Xamarin.Mac/mscorlib.dll";
var compilation = CSharpCompilation.CreateScriptCompilation (
    assemblyName: "Script",
    syntaxTree: CSharpSyntaxTree.ParseText (
        text: "true",
        options: new CSharpParseOptions (kind: SourceCodeKind.Script)),
    references: new [] { MetadataReference.CreateFromFile (corlibPath) },
    options: new CSharpCompilationOptions (OutputKind.DynamicallyLinkedLibrary));
Debug.Assert (compilation.GetDiagnostics ().Length == 0);

This will result in the diagnostic:

(1,1): error CS0400: The type or namespace name 'System.Object, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' could not be found in the global namespace (are you missing an assembly reference?)

This is due to the compilation falling back (because returnType: is not specified via CreateScriptCompilation) to typeof(object) for SynthesizedInteractiveInitializerMethod's Task<> return type type argument, which will be the host corlib's System.Object and not the System.Object from the Xamarin.iOS (or any other non-host-compatible) corlib.

[ManishJayaswal]

#8507 will hopefully resolve this issue.

[abock]

I've just finally gotten around to updating #8507 to account for the last round of feedback and rebased it on today's master. I hope we can get this merged. We need this in Xamarin Workbooks and have been shipping a patched version of Roslyn with the patch since February.