Nullability of type parameters "where T : class?"

ICSharpCode.Decompiler/CSharp/Syntax/TypeSystemAstBuilder.cs

@@ -1724,7 +1724,11 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			Constraint c = new Constraint();
 			c.TypeParameter = new SimpleType (tp.Name);
 			if (tp.HasReferenceTypeConstraint) {
-				c.BaseTypes.Add(new PrimitiveType("class"));
+				if (tp.NullabilityConstraint == Nullability.Nullable) {
+					c.BaseTypes.Add(new PrimitiveType("class").MakeNullableType());
+				} else {
+					c.BaseTypes.Add(new PrimitiveType("class"));
+				}
 			} else if (tp.HasValueTypeConstraint) {
 				c.BaseTypes.Add(new PrimitiveType("struct"));
 			}

ICSharpCode.Decompiler/TypeSystem/ITypeParameter.cs

@@ -86,6 +86,11 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// Gets if the type parameter has the 'struct' constraint.
 		/// </summary>
 		bool HasValueTypeConstraint { get; }
+
+		/// <summary>
+		/// Nullability of the reference type constraint. (e.g. "where T : class?").
+		/// </summary>
+		Nullability NullabilityConstraint { get; }
 	}
 	
 	/// <summary>

ICSharpCode.Decompiler/TypeSystem/Implementation/AbstractTypeParameter.cs

@@ -158,7 +158,8 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		public abstract bool HasDefaultConstructorConstraint { get; }
 		public abstract bool HasReferenceTypeConstraint { get; }
 		public abstract bool HasValueTypeConstraint { get; }
-		
+		public abstract Nullability NullabilityConstraint { get; }
+
 		public TypeKind Kind {
 			get { return TypeKind.TypeParameter; }
 		}

ICSharpCode.Decompiler/TypeSystem/Implementation/DefaultTypeParameter.cs

@@ -26,6 +26,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		readonly bool hasValueTypeConstraint;
 		readonly bool hasReferenceTypeConstraint;
 		readonly bool hasDefaultConstructorConstraint;
+		readonly Nullability nullabilityConstraint;
 		readonly IReadOnlyList<IType> constraints;
 		readonly IReadOnlyList<IAttribute> attributes;
 
@@ -35,12 +36,13 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			VarianceModifier variance = VarianceModifier.Invariant,
 			IReadOnlyList<IAttribute> attributes = null,
 			bool hasValueTypeConstraint = false, bool hasReferenceTypeConstraint = false, bool hasDefaultConstructorConstraint = false,
-			IReadOnlyList<IType> constraints = null)
+			IReadOnlyList<IType> constraints = null, Nullability nullabilityConstraint = Nullability.Oblivious)
 			: base(owner, index, name, variance)
 		{
 			this.hasValueTypeConstraint = hasValueTypeConstraint;
 			this.hasReferenceTypeConstraint = hasReferenceTypeConstraint;
 			this.hasDefaultConstructorConstraint = hasDefaultConstructorConstraint;
+			this.nullabilityConstraint = nullabilityConstraint;
 			this.constraints = constraints ?? EmptyList<IType>.Instance;
 			this.attributes = attributes ?? EmptyList<IAttribute>.Instance;
 		}
@@ -51,12 +53,13 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			VarianceModifier variance = VarianceModifier.Invariant,
 			IReadOnlyList<IAttribute> attributes = null,
 			bool hasValueTypeConstraint = false, bool hasReferenceTypeConstraint = false, bool hasDefaultConstructorConstraint = false,
-			IReadOnlyList<IType> constraints = null)
+			IReadOnlyList<IType> constraints = null, Nullability nullabilityConstraint = Nullability.Oblivious)
 			: base(compilation, ownerType, index, name, variance)
 		{
 			this.hasValueTypeConstraint = hasValueTypeConstraint;
 			this.hasReferenceTypeConstraint = hasReferenceTypeConstraint;
 			this.hasDefaultConstructorConstraint = hasDefaultConstructorConstraint;
+			this.nullabilityConstraint = nullabilityConstraint;
 			this.constraints = constraints ?? EmptyList<IType>.Instance;
 			this.attributes = attributes ?? EmptyList<IAttribute>.Instance;
 		}
@@ -66,6 +69,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		public override bool HasValueTypeConstraint => hasValueTypeConstraint;
 		public override bool HasReferenceTypeConstraint => hasReferenceTypeConstraint;
 		public override bool HasDefaultConstructorConstraint => hasDefaultConstructorConstraint;
+		public override Nullability NullabilityConstraint => nullabilityConstraint;
 
 		public override IEnumerable<IType> DirectBaseTypes {
 			get {

ICSharpCode.Decompiler/TypeSystem/Implementation/DummyTypeParameter.cs

@@ -174,5 +174,9 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		bool ITypeParameter.HasValueTypeConstraint {
 			get { return false; }
 		}
+
+		Nullability ITypeParameter.NullabilityConstraint {
+			get { return Nullability.Oblivious; }
+		}
 	}
 }

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataTypeParameter.cs

@@ -35,6 +35,8 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 
 		// lazy-loaded:
 		IReadOnlyList<IType> constraints;
+		const byte nullabilityNotYetLoaded = 255;
+		byte nullabilityConstraint = nullabilityNotYetLoaded;
 
 		public static ITypeParameter[] Create(MetadataModule module, ITypeDefinition copyFromOuter, IEntity owner, GenericParameterHandleCollection handles)
 		{
@@ -112,6 +114,37 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		public override bool HasReferenceTypeConstraint => (attr & GenericParameterAttributes.ReferenceTypeConstraint) != 0;
 		public override bool HasValueTypeConstraint => (attr & GenericParameterAttributes.NotNullableValueTypeConstraint) != 0;
 
+		public override Nullability NullabilityConstraint {
+			get {
+				if (nullabilityConstraint == nullabilityNotYetLoaded) {
+					nullabilityConstraint = (byte)LoadNullabilityConstraint();
+				}
+				return (Nullability)nullabilityConstraint;
+			}
+		}
+
+		Nullability LoadNullabilityConstraint()
+		{
+			if ((module.TypeSystemOptions & TypeSystemOptions.NullabilityAnnotations) == 0)
+				return Nullability.Oblivious;
+
+			var metadata = module.metadata;
+			var gp = metadata.GetGenericParameter(handle);
+
+			foreach (var handle in gp.GetCustomAttributes()) {
+				var customAttribute = metadata.GetCustomAttribute(handle);
+				if (customAttribute.IsKnownAttribute(metadata, KnownAttribute.Nullable)) {
+					var attrVal = customAttribute.DecodeValue(module.TypeProvider);
+					if (attrVal.FixedArguments.Length == 1) {
+						if (attrVal.FixedArguments[0].Value is byte b && b <= 2) {
+							return (Nullability)b;
+						}
+					}
+				}
+			}
+			return Nullability.Oblivious;
+		}
+
 		public override IEnumerable<IType> DirectBaseTypes {
 			get {
 				var constraints = LazyInit.VolatileRead(ref this.constraints);

ICSharpCode.Decompiler/TypeSystem/Implementation/SpecializedMethod.cs

@@ -253,7 +253,9 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			public override bool HasDefaultConstructorConstraint {
 				get { return baseTp.HasDefaultConstructorConstraint; }
 			}
-			
+
+			public override Nullability NullabilityConstraint => baseTp.NullabilityConstraint;
+
 			public override IEnumerable<IType> DirectBaseTypes {
 				get {
 					return baseTp.DirectBaseTypes.Select(t => t.AcceptVisitor(substitution));