I created an empty ASP.Net MVC 5 project in Visual Studio 2015 recently.
This project targeted .Net Framework version 4.5.2 by default. The project compiled successfully and ran as expected without any issues.

Later on, I changed this from the project properties to target the latest .Net Framework v4.6
This time, however, when I tried running the project it failed with the following error.

C:\Program Files (x86)\IIS Express> "C:\Windows\Microsoft.NET\Framework\v4.0.30319\csc.exe" /t:library /utf8output /R:"C:\Users\ckurz\AppData\Local\Temp\Temporary ASP.NET Files\vs\af543f55\865ca3e2\assembly\dl3\ab86a7ca\9447cc05_9ac4d001\System.Web.Optimization.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\Microsoft.CSharp\v4.0_4.0.0.0__b03f5f7f11d50a3a\Microsoft.CSharp.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_32\System.EnterpriseServices\v4.0_4.0.0.0__b03f5f7f11d50a3a\System.EnterpriseServices.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.ServiceModel.Activities\v4.0_4.0.0.0__31bf3856ad364e35\System.ServiceModel.Activities.dll" /R:"C:\Users\ckurz\AppData\Local\Temp\Temporary ASP.NET Files\vs\af543f55\865ca3e2\assembly\dl3\3a5fd52a\fe1de005_9ac4d001\System.Web.Mvc.dll" /R:"C:\Users\ckurz\AppData\Local\Temp\Temporary ASP.NET Files\vs\af543f55\865ca3e2\assembly\dl3\4ec01510\6e387621_9ac4d001\mvc452.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.ComponentModel.DataAnnotations\v4.0_4.0.0.0__31bf3856ad364e35\System.ComponentModel.DataAnnotations.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Core\v4.0_4.0.0.0__b77a5c561934e089\System.Core.dll" /R:"C:\Users\ckurz\AppData\Local\Temp\Temporary ASP.NET Files\vs\af543f55\865ca3e2\assembly\dl3\d21f1f2b\cf2dcf05_9ac4d001\System.Web.WebPages.Razor.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.Services\v4.0_4.0.0.0__b03f5f7f11d50a3a\System.Web.Services.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.WebPages.Deployment\v4.0_1.0.0.0__31bf3856ad364e35\System.Web.WebPages.Deployment.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.DynamicData\v4.0_4.0.0.0__31bf3856ad364e35\System.Web.DynamicData.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Configuration\v4.0_4.0.0.0__b03f5f7f11d50a3a\System.Configuration.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\Microsoft.VisualStudio.Web.PageInspector.Loader\v4.0_1.0.0.0__b03f5f7f11d50a3a\Microsoft.VisualStudio.Web.PageInspector.Loader.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_32\System.Web\v4.0_4.0.0.0__b03f5f7f11d50a3a\System.Web.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.ApplicationServices\v4.0_4.0.0.0__31bf3856ad364e35\System.Web.ApplicationServices.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.ServiceModel\v4.0_4.0.0.0__b77a5c561934e089\System.ServiceModel.dll" /R:"C:\Users\ckurz\AppData\Local\Temp\Temporary ASP.NET Files\vs\af543f55\865ca3e2\assembly\dl3\5b520775\2a2b4106_9ac4d001\Microsoft.CodeDom.Providers.DotNetCompilerPlatform.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Data.DataSetExtensions\v4.0_4.0.0.0__b77a5c561934e089\System.Data.DataSetExtensions.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.Extensions\v4.0_4.0.0.0__31bf3856ad364e35\System.Web.Extensions.dll" /R:"C:\Users\ckurz\AppData\Local\Temp\Temporary ASP.NET Files\vs\af543f55\865ca3e2\assembly\dl3\517d2319\377ff305_9ac4d001\Antlr3.Runtime.dll" /R:"C:\Users\ckurz\AppData\Local\Temp\Temporary ASP.NET Files\vs\af543f55\865ca3e2\assembly\dl3\b132331e\aedfce05_9ac4d001\System.Web.WebPages.Deployment.dll" /R:"C:\Windows\Microsoft.NET\Framework\v4.0.30319\mscorlib.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.IdentityModel\v4.0_4.0.0.0__b77a5c561934e089\System.IdentityModel.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\Microsoft.Web.Infrastructure\v4.0_1.0.0.0__31bf3856ad364e35\Microsoft.Web.Infrastructure.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.ServiceModel.Web\v4.0_4.0.0.0__31bf3856ad364e35\System.ServiceModel.Web.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Xml.Linq\v4.0_4.0.0.0__b77a5c561934e089\System.Xml.Linq.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.WorkflowServices\v4.0_4.0.0.0__31bf3856ad364e35\System.WorkflowServices.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System\v4.0_4.0.0.0__b77a5c561934e089\System.dll" /R:"C:\Users\ckurz\AppData\Local\Temp\Temporary ASP.NET Files\vs\af543f55\865ca3e2\assembly\dl3\0af4374b\7aafc905_9ac4d001\System.Web.Razor.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Xml\v4.0_4.0.0.0__b77a5c561934e089\System.Xml.dll" /R:"C:\Users\ckurz\AppData\Local\Temp\Temporary ASP.NET Files\vs\af543f55\865ca3e2\assembly\dl3\b964c8b6\30c9f505_9ac4d001\WebGrease.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Drawing\v4.0_4.0.0.0__b03f5f7f11d50a3a\System.Drawing.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Runtime.Serialization\v4.0_4.0.0.0__b77a5c561934e089\System.Runtime.Serialization.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.ServiceModel.Activation\v4.0_4.0.0.0__31bf3856ad364e35\System.ServiceModel.Activation.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.WebPages.Deployment\v4.0_2.0.0.0__31bf3856ad364e35\System.Web.WebPages.Deployment.dll" /R:"C:\Users\ckurz\AppData\Local\Temp\Temporary ASP.NET Files\vs\af543f55\865ca3e2\assembly\dl3\f5630b78\8d91ce05_9ac4d001\System.Web.Helpers.dll" /R:"C:\Users\ckurz\AppData\Local\Temp\Temporary ASP.NET Files\vs\af543f55\865ca3e2\assembly\dl3\fe5a9f88\be06cf05_9ac4d001\System.Web.WebPages.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_32\System.Data\v4.0_4.0.0.0__b77a5c561934e089\System.Data.dll" /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Activities\v4.0_4.0.0.0__31bf3856ad364e35\System.Activities.dll" /R:"C:\Users\ckurz\AppData\Local\Temp\Temporary ASP.NET Files\vs\af543f55\865ca3e2\assembly\dl3\cc0337ba\8b692306_9ac4d001\Newtonsoft.Json.dll" /out:"C:\Users\ckurz\AppData\Local\Temp\Temporary ASP.NET Files\vs\af543f55\865ca3e2\App_global.asax.y7q4zs_7.dll" /D:DEBUG /debug+ /optimize- /warnaserror /w:4 /nowarn:1659;1699;1701;612;618 /langversion:6 /nowarn:1659;1699;1701 "C:\Users\ckurz\AppData\Local\Temp\Temporary ASP.NET Files\vs\af543f55\865ca3e2\App_global.asax.y7q4zs_7.0.cs" "C:\Users\ckurz\AppData\Local\Temp\Temporary ASP.NET Files\vs\af543f55\865ca3e2\App_global.asax.y7q4zs_7.1.cs"

Microsoft (R) Visual C# Compiler version 4.6.0081.0

for C# 5 Copyright (C) Microsoft Corporation. All rights reserved.

This compiler is provided as part of the Microsoft (R) .NET Framework, but only supports language versions up to C# 5, which is no longer the latest version. For compilers that support newer versions of the C# programming language, see http://go.microsoft.com/fwlink/?LinkID=533240

error CS1617: Invalid option '6' for /langversion; must be ISO-1, ISO-2, 3, 4, 5 or Default

This was quite weird.
After searching a bit on Google I finally found the following solution.
In the web.config file there is a section system.codedom as follows...

<system.codedom>
    <compilers>
      <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CSharp.CSharpCodeProvider, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
        <providerOption name="CompilerVersion" value="v4.0" />
      </compiler>
      <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:14 /nowarn:41008 /define:_MYTYPE=\&amp;quot;Web\&amp;quot; /optionInfer+" />
    </compilers>
</system.codedom>

Observe the value of the compiler type for the C# language.
It needs to be changed from Microsoft.CSharp.CSharpCodeProvider, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 to Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35

In a nutshell,

• for 4.5 and C#5 => type="Microsoft.CSharp.CSharpCodeProvider"

• for 4.6 and C#6 => type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider"

I'm not yet sure why this issue occurred in the first place, I'm guessing this could be a bug in Visual Studio 2015.