Are the Roslyn checks still necessary?

[stephentoub]

In a variety of places, we check MSBuildToolsVersion to determine whether we're using Roslyn, e.g.

buildtools/src/Microsoft.DotNet.Build.Tasks/PackageFiles/ReferenceAssemblies.targets

Line 9 in e4b8152

<_usingRoslyn Condition="'$(MSBuildToolsVersion)' == 'dogfood' OR '$(MSBuildToolsVersion)' == 'Current' OR '$(MSBuildToolsVersion)' &gt;= '14.0'">true</_usingRoslyn>

and

buildtools/src/Microsoft.DotNet.Build.Tasks/PackageFiles/Build.Common.props

Lines 48 to 55 in 28606b8

	<RoslynIncompatibleMsbuildVersion Condition="'$(MSBuildToolsVersion)' == '2.0' OR
	'$(MSBuildToolsVersion)' == '3.5' OR
	'$(MSBuildToolsVersion)' == '4.0' OR
	'$(MSBuildToolsVersion)' == '12.0' OR
	'$(MSBuildToolsVersion)' == '14.0'">true</RoslynIncompatibleMsbuildVersion>
	<!-- UseSharedCompilation is not supported in MSBuild 14.0 with the new version of roslyn, so make sure we are not using that for building -->
	<UseSharedCompilation Condition="'$(UseSharedCompilation)' == '' and '$(UseRoslynCompilers)' != 'false' and '$(RoslynIncompatibleMsbuildVersion)' != 'true'">true</UseSharedCompilation>
	<GenFacadesIgnoreBuildAndRevisionMismatch>true</GenFacadesIgnoreBuildAndRevisionMismatch>

Is this still necessary? Or can we just assume we're using Roslyn everywhere and simplify these checks?

[ViktorHofer]

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