/repos/source-build-reference-packages/artifacts/source-build/self/src/src/referencePackages/src/microsoft.codeanalysis.csharp/4.0.1/lib/netstandard2.0/Microsoft.CodeAnalysis.CSharp.cs(2820,34): error CS0453: The type 'TResult' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'Nullable<T>' [/repos/source-build-reference-packages/artifacts/source-build/self/src/src/referencePackages/src/microsoft.codeanalysis.csharp/4.0.1/Microsoft.CodeAnalysis.CSharp.4.0.1.csproj] [/root/.nuget/packages/microsoft.dotnet.arcade.sdk/7.0.0-beta.22376.3/tools/Build.proj]
  /repos/source-build-reference-packages/artifacts/source-build/self/src/src/referencePackages/src/microsoft.codeanalysis.csharp/4.0.1/lib/netstandard2.0/Microsoft.CodeAnalysis.CSharp.cs(2841,34): error CS0508: 'AccessorListSyntax.Accept<TResult>(CSharpSyntaxVisitor<TResult>)': return type must be 'TResult' to match overridden member 'CSharpSyntaxNode.Accept<TResult>(CSharpSyntaxVisitor<TResult>)' [/repos/source-build-reference-packages/artifacts/source-build/self/src/src/referencePackages/src/microsoft.codeanalysis.csharp/4.0.1/Microsoft.CodeAnalysis.CSharp.4.0.1.csproj] [/root/.nuget/packages/microsoft.dotnet.arcade.sdk/7.0.0-beta.22376.3/tools/Build.proj]

public override TResult? Accept<TResult>(CSharpSyntaxVisitor<TResult> visitor)

The addition of nullable reference types introduces a potential ambiguity in the meaning of T? in generic methods. If T is a struct, T? is the same as [System.Nullable<T>](https://docs.microsoft.com/en-us/dotnet/api/system.nullable-1). However, if T is a reference type, T? means that null is a valid value. The ambiguity arises because overriding methods can't include constraints. The new default constraint resolves this ambiguity.