Don't suggest an upgrade to an unsupported state

src/Features/CSharp/Portable/UpgradeProject/CSharpUpgradeProjectCodeFixProvider.cs

@@ -85,8 +85,11 @@ public override bool IsUpgrade(ParseOptions projectOptions, string newVersion)
             var parseOptions = (CSharpParseOptions)projectOptions;
             Contract.ThrowIfFalse(LanguageVersionFacts.TryParse(newVersion, out var parsedNewVersion));
 
+            var mappedVersion = parsedNewVersion.MapSpecifiedToEffectiveVersion();
+
             // treat equivalent versions (one generic and one specific) to be a valid upgrade
-            return parsedNewVersion.MapSpecifiedToEffectiveVersion() >= parseOptions.LanguageVersion &&
+            return mappedVersion >= parseOptions.LanguageVersion &&
+                mappedVersion < LanguageVersion.CSharp8 &&
                 parseOptions.SpecifiedLanguageVersion.ToDisplayString() != newVersion;
         }
     }

src/Features/Core/Portable/UpgradeProject/AbstractUpgradeProjectCodeFixProvider.cs

@@ -39,15 +39,24 @@ protected ImmutableArray<CodeAction> GetUpgradeProjectCodeActionsAsync(CodeFixCo
             var project = context.Document.Project;
             var solution = project.Solution;
             var newVersion = SuggestedVersion(context.Diagnostics);
+
             var result = new List<CodeAction>();
             var language = project.Language;
 
+            var upgradeableProjects = solution.Projects.Where(p => CanUpgrade(p, language, newVersion)).AsImmutable();
+
+            if (upgradeableProjects.Length == 0)
+            {
+                return ImmutableArray<CodeAction>.Empty;
+            }
+
             var fixOneProjectTitle = string.Format(UpgradeThisProjectResource, newVersion);
             var fixOneProject = new ProjectOptionsChangeAction(fixOneProjectTitle,
                 _ => Task.FromResult(UpgradeProject(project, newVersion)));
 
             result.Add(fixOneProject);
-            if (solution.Projects.Count(p => CanUpgrade(p, language, newVersion)) > 1)
+
+            if (upgradeableProjects.Length > 1)
             {
                 var fixAllProjectsTitle = string.Format(UpgradeAllProjectsResource, newVersion);