Internal MSBuild error when --maxcpucount:8 supplied in cmdline (dotnet build)

[wingrime]

dotnet build  -c Release --framework:net5.0 --maxcpucount:8

gives:

MSBUILD : error MSB1025: внутренняя ошибка при выполнении MSBuild.
Microsoft.Build.Shared.InternalErrorException: MSB0001: Internal MSBuild Error: The switch name extracted from either the partially or completely unquoted arg should be the same.
   at Microsoft.Build.Shared.ErrorUtilities.ThrowInternalError(String message, Exception innerException, Object[] args) in MSBuild.dll:token 0x60002e4+0x14
   at Microsoft.Build.CommandLine.MSBuildApp.ExtractSwitchParameters(String commandLineArg, String unquotedCommandLineArg, Int32 doubleQuotesRemovedFromArg, String switchName, Int32 switchParameterIndicator) in MSBuild.dll:token 0x6000097+0x19
   at Microsoft.Build.CommandLine.MSBuildApp.GatherCommandLineSwitches(List`1 commandLineArgs, CommandLineSwitches commandLineSwitches) in MSBuild.dll:token 0x6000096+0xa2
   at Microsoft.Build.CommandLine.MSBuildApp.GatherAllSwitches(String[] commandLine, CommandLineSwitches& switchesFromAutoResponseFile, CommandLineSwitches& switchesNotFromAutoResponseFile) in MSBuild.dll:token 0x6000095+0x46
   at Microsoft.Build.CommandLine.MSBuildApp.Execute(String[] commandLine) in MSBuild.dll:token 0x600008a+0x10f
Unhandled exception: Microsoft.Build.Shared.InternalErrorException: MSB0001: Internal MSBuild Error: The switch name extracted from either the partially or completely unquoted arg should be the same.
   at Microsoft.Build.Shared.ErrorUtilities.ThrowInternalError(String message, Exception innerException, Object[] args) in MSBuild.dll:token 0x60002e4+0x14
   at Microsoft.Build.CommandLine.MSBuildApp.ExtractSwitchParameters(String commandLineArg, String unquotedCommandLineArg, Int32 doubleQuotesRemovedFromArg, String switchName, Int32 switchParameterIndicator) in MSBuild.dll:token 0x6000097+0x19
   at Microsoft.Build.CommandLine.MSBuildApp.GatherCommandLineSwitches(List`1 commandLineArgs, CommandLineSwitches commandLineSwitches) in MSBuild.dll:token 0x6000096+0xa2
   at Microsoft.Build.CommandLine.MSBuildApp.GatherAllSwitches(String[] commandLine, CommandLineSwitches& switchesFromAutoResponseFile, CommandLineSwitches& switchesNotFromAutoResponseFile) in MSBuild.dll:token 0x6000095+0x46
   at Microsoft.Build.CommandLine.MSBuildApp.Execute(String[] commandLine) in MSBuild.dll:token 0x600008a+0x10f
   at Microsoft.Build.CommandLine.MSBuildApp.Main(String[] args) in MSBuild.dll:token 0x6000086+0x22
   at Microsoft.DotNet.Cli.Utils.MSBuildForwardingAppWithoutLogging.ExecuteInProc(String[] arguments) in Microsoft.DotNet.Cli.Utils.dll:token 0x6000106+0x62

but dotnet build -c Release --framework:net5.0 -maxcpucount:8 build project as expected

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[rainersigwald]

Looks like only the validation is wrong:

msbuild/src/MSBuild/XMake.cs

Lines 1788 to 1789 in 48ffc98

	ErrorUtilities.VerifyThrow(switchName == unquotedSwitchIndicatorAndName.Substring(1),
	"The switch name extracted from either the partially or completely unquoted arg should be the same.");

That doesn't account for the double-hyphen support added in #5786.