Script compilations created from a previous compilation have incorrect return type

[abock]

We are now finally able to use the Roslyn 2.0 NuGet for Xamarin Workbooks, which means I can finally leverage my work for supporting compilations against a target (not host) corlib.

Unfortunately the tests I wrote for that fix were not extensive enough to catch an issue where subsequent compilations result in an implicit reference to host corlib and not the target corlib.

This is due to the implementation of ScriptCompilationInfo.ReturnType, which was desired to never return null:

public Type ReturnType => ReturnTypeOpt ?? typeof(object);

I did not catch all the places in Roslyn that were using ReturnType but should now be using ReturnTypeOpt:

• CSharpScriptCompilationInfo.WithPreviousScriptCompilation
• CSharpCompilation.WithScriptCompilationInfo
• VisualBasicScriptCompilationInfo.WithPreviousScriptCompilation
• VisualBasicCompilation.WithScriptCompilationInfo

This is how the bug ends up manifesting in Workbooks: