In VisualFSharp.sln, I pressed F12 on CheckFileInProject at the following line in CompletionProvider.fs,

    let! checkFileAnswer = FSharpChecker.Instance.CheckFileInProject(parseResults, filePath, textVersionHash, sourceText.ToString(), options)

An assert happened telling me that First failed on this line:

                let refDocumentId = document.Project.Solution.GetDocumentIdsWithFilePath(range.FileName).First()

The fix for the assert should be to check if the array of documents is empty, However I don't particularly understand why this goto-definition action would fail.