Scaffolding fails without details on why, when using certain c# language syntax

Environment:

• ASP.Net Core 2.1
• Unspecificed C# language in project settings, I believe I earlier had downloaded and “installed” a newer c# (7.3) version. Then removed it from the project because of then also scaffolding failing.
• Visual Studio Community 2017, Windows

This was one I really did not like guys! Since the error gives no clue about what is going on I had to backtrack my commits and guess which change broke scaffolding, thankfully i recalled that scaffolding has some internal build mechanism not using same c# version as the current project, and is therefor sensitive to certain c# language syntax, which I assumes is new c# syntax.

Finding the generator 'razorpage'...
Running the generator 'razorpage'...
Attempting to compile the application in memory.
   at Microsoft.VisualStudio.Web.CodeGeneration.ActionInvoker.<BuildCommandLine>b__6_0()
   at Microsoft.Extensions.CommandLineUtils.CommandLineApplication.Execute(String[] args)
   at Microsoft.VisualStudio.Web.CodeGeneration.ActionInvoker.Execute(String[] args)
   at Microsoft.VisualStudio.Web.CodeGeneration.CodeGenCommand.Execute(String[] args)

The problematic code was using implied default value default vs default(CancellationToken)

Does NOT work:

public async override Task<int> SaveChangesAsync(bool acceptAllChangesOnSuccess, CancellationToken cancellationToken = default)
{
  ...
}

Does work:

public async override Task<int> SaveChangesAsync(bool acceptAllChangesOnSuccess, CancellationToken cancellationToken = default(CancellationToken))
{
   ...
}

Suggested fixes/changes from a user perspective:

• (MUST) Validate, then give better error message that code failed validation according to scaffolding build mechanism.
• (MAYBE) Have scaffolding build with same c# version as project (I am on a limb here, I might have missunderstood the tech)

[Preecington]

If anyone needs a quick fix to this before the PR merge goes out into an official package, I found installing the following packages makes the scaffolding work again:

<PackageReference Include="Microsoft.CodeAnalysis.Common" Version="3.2.0-beta3-final" />
<PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="3.2.0-beta3-final" />
<PackageReference Include="Microsoft.CodeAnalysis.CSharp.Workspaces" Version="3.2.0-beta3-final" />

[OrihuelaConde]

If anyone needs a quick fix to this before the PR merge goes out into an official package, I found installing the following packages makes the scaffolding work again:

<PackageReference Include="Microsoft.CodeAnalysis.Common" Version="3.2.0-beta3-final" />
<PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="3.2.0-beta3-final" />
<PackageReference Include="Microsoft.CodeAnalysis.CSharp.Workspaces" Version="3.2.0-beta3-final" />

Thanks, you can fix this only with one package (its include the rest of the needed packages )

<PackageReference Include="Microsoft.CodeAnalysis.CSharp.Workspaces" Version="3.3.1" />

We are using EF Core 2.2.6 and this issue is not fixed.