roslyn: TreatWarningsAsErrors ignored when severity set in EditorConfig

Workaround

Use dotnet build -warnaserror instead of dotnet build -p:TreatWarningsAsErrors=true in step 5 of the below repros.

Repro 1 (Compiler diagnostics)

Repro steps

1. dotnet new console
2. Edit the source code to:

class Program
{
    static void Main(string[] args)
    {
        // warning CS0219: The variable 'unused' is assigned but its value is never used
        int unused = 0;
    }
}

1. dotnet build -p:TreatWarningsAsErrors=true
2. Add .editorconfig with:

root = true

[*.cs]
dotnet_diagnostic.CS0219.severity = warning

1. dotnet build -p:TreatWarningsAsErrors=true

Expected behavior

I expect the build output in steps 3 and 5 to be the same, i.e. the build in both cases should fail due to CS0219 being promoted to an error.

Actual behavior

Build fails in step 3, but not in step 5. dotnet_diagnostic.CS0219.severity = warning takes precedence over /warnaserror+ passed to csc.exe from -p:TreatWarningsAsErrors=true

Repro 2 (Analyzer diagnostics)

Analyzer package

Microsoft.CodeAnalysis.FxCopAnalyzers

Package Version

v2.9.8 (Latest)

Diagnostic ID

N/A

$ dotnet --info
.NET Core SDK (reflecting any global.json):
 Version:   3.1.200
 Commit:    c5123d973b

Runtime Environment:
 OS Name:     ubuntu
 OS Version:  18.04
 OS Platform: Linux
 RID:         ubuntu.18.04-x64
 Base Path:   /usr/share/dotnet/sdk/3.1.200/

Host (useful for support):
  Version: 3.1.2
  Commit:  916b5cba26

.NET Core SDKs installed:
  3.1.200 [/usr/share/dotnet/sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.App 3.1.2 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 3.1.2 [/usr/share/dotnet/shared/Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
  https://aka.ms/dotnet-download

Repro steps

1. dotnet new console
2. dotnet add package Microsoft.CodeAnalysis.FxCopAnalyzers -v 2.9.8
3. dotnet build -p:TreatWarningsAsErrors=true
4. Add .editorconfig with:

root = true

[*.cs]
dotnet_diagnostic.CA1303.severity = default
dotnet_diagnostic.CA1801.severity = warning

1. dotnet build -p:TreatWarningsAsErrors=true

Expected behavior

I expect the build output in steps 3 and 5 to be the same, i.e. the build in both cases should fail due to two build errors: CA1303, CA1801.

Actual behavior

The output from step 3 shows build failure due to two errors: CA1303, CA1801. This is correct and expected:

$ dotnet build -p:TreatWarningsAsErrors=true
Microsoft (R) Build Engine version 16.5.0+d4cbfca49 for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.

  Restore completed in 35.32 ms for /home/a-x-d/dev/fx1/fx1.csproj.
Program.cs(9,31): error CA1303: Method 'void Program.Main(string[] args)' passes a literal string as parameter 'value' of a call to 'void Console.WriteLine(string value)'. Retrieve the following string(s) from a resource table instead: "Hello World!". [/home/a-x-d/dev/fx1/fx1.csproj]
Program.cs(7,35): error CA1801: Parameter args of method Main is never used. Remove the parameter or use it in the method body. [/home/a-x-d/dev/fx1/fx1.csproj]

Build FAILED.

Program.cs(9,31): error CA1303: Method 'void Program.Main(string[] args)' passes a literal string as parameter 'value' of a call to 'void Console.WriteLine(string value)'. Retrieve the following string(s) from a resource table instead: "Hello World!". [/home/a-x-d/dev/fx1/fx1.csproj]
Program.cs(7,35): error CA1801: Parameter args of method Main is never used. Remove the parameter or use it in the method body. [/home/a-x-d/dev/fx1/fx1.csproj]
    0 Warning(s)
    2 Error(s)

Time Elapsed 00:00:00.98

The build output from step 5 shows build passing with two warnings for CA1303, CA1801. Somehow editor config configuration overrides TreatWarningsAsErrors property. This is not expected and seems incorrect:

$ dotnet build -p:TreatWarningsAsErrors=true
Microsoft (R) Build Engine version 16.5.0+d4cbfca49 for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.

  Restore completed in 216.45 ms for /home/a-x-d/dev/fx1/fx1.csproj.
Program.cs(9,31): warning CA1303: Method 'void Program.Main(string[] args)' passes a literal string as parameter 'value' of a call to 'void Co
nsole.WriteLine(string value)'. Retrieve the following string(s) from a resource table instead: "Hello World!". [/home/a-x-d/dev/fx1/fx1.cspro
j]
Program.cs(7,35): warning CA1801: Parameter args of method Main is never used. Remove the parameter or use it in the method body. [/home/a-x-d
/dev/fx1/fx1.csproj]
  fx1 -> /home/a-x-d/dev/fx1/bin/Debug/netcoreapp3.1/fx1.dll

Build succeeded.

Program.cs(9,31): warning CA1303: Method 'void Program.Main(string[] args)' passes a literal string as parameter 'value' of a call to 'void Co
nsole.WriteLine(string value)'. Retrieve the following string(s) from a resource table instead: "Hello World!". [/home/a-x-d/dev/fx1/fx1.cspro
j]
Program.cs(7,35): warning CA1801: Parameter args of method Main is never used. Remove the parameter or use it in the method body. [/home/a-x-d/dev/fx1/fx1.csproj]
    2 Warning(s)
    0 Error(s)

Time Elapsed 00:00:01.35