Need to override the StandardErrorLoggingImportance property as well

        /// <summary>
        /// Standard error output should be considered of high importance so that it shows up in even 
        /// minimal logs. This is how unhandled exception information is output by the runtime.
        /// </summary>
        protected override MessageImportance StandardErrorLoggingImportance => MessageImportance.High;

@nguerrera Would you have some tips for setting up a CLI with a custom-build version of Roslyn, so that I can validate the fix?

I think the easiest thing would be to produce the Microsoft.NET.Compiler.Core nupkg with your change and test using RTM CLI and a package ref to your nupkg.

For a more out-of-the-box-like test, you could also download the zip https://www.microsoft.com/net/download/thank-you/dotnet-sdk-2.1.300-windows-x86-binaries and replace the sdk\2.1.300\Roslyn directory with your bits.

@jcouv I validated this change locally. Here is the steps I took to validating.

1. In roslyn enlistment ran: powershell -noprofile -executionPolicy RemoteSigned -file build\scripts\build.ps1 -bootstrap -buildcoreclr. That puts a valid MSBuild layout into "E:\code\roslyn\Binaries\Bootstrap\Microsoft.NETCore.Compilers\42.42.42.42\tools"
2. Created a Net Core app project that had the stack overflow case.
3. In the sample code directory
   1. Ran dotnet build and verified no stack printed
   2. Ran dotnet build /p:RoslynTargetsPath=E:\code\roslyn\Binaries\Bootstrap\Microsoft.NETCore.Compilers\42.42.42.42\tools and verified the stack trace was printed

@jaredpar If I understood correctly, Rainer's comment (see linked issue) indicated that only one of the fixes should be sufficient. I'll test and confirm.

Verified the fix using Jared's instructions. Now, the error is printed out (including the very long stacktrace in the overflow scenario).

c:\...\Microsoft.CSharp.Core.targets(52,5): error : FailFast: [C:\repos\overflow\overflow.csproj]
c:\...\Microsoft.CSharp.Core.targets(52,5): error : System.InsufficientExecutionStackException: Insufficient stack to continue executing the program safely. This can happen from having too many functions on the call stack or function on the stack using too much stack space. [C:\repos\overflow\overflow.csproj]
c:\...\Microsoft.CSharp.Core.targets(52,5): error :    at System.Runtime.CompilerServices.RuntimeHelpers.EnsureSufficientExecutionStack() [C:\repos\overflow\overflow.csproj]
c:\...\Microsoft.CSharp.Core.targets(52,5): error :    at System.Runtime.CompilerServices.RuntimeHelpers.EnsureSufficientExecutionStack() [C:\repos\overflow\overflow.csproj]
c:\...\Microsoft.CSharp.Core.targets(52,5): error :    at Microsoft.CodeAnalysis.StackGuard.EnsureSufficientExecutionStack(Int32 recursionDepth) [C:\repos\overflow\overflow.csproj]
[... the stack for the overflow follows ...]

@jaredpar for second sign-off and for ask-mode approval for 15.8. Thanks