TypeInference: add support for nullability annotated types

ICSharpCode.Decompiler/CSharp/Resolver/TypeInference.cs

@@ -395,11 +395,10 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		
 		static IMethod GetDelegateOrExpressionTreeSignature(IType t)
 		{
-			ParameterizedType pt = t as ParameterizedType;
-			if (pt != null && pt.TypeParameterCount == 1 && pt.Name == "Expression"
-			    && pt.Namespace == "System.Linq.Expressions")
+			if (t.TypeParameterCount == 1 && t.Name == "Expression"
+			    && t.Namespace == "System.Linq.Expressions")
 			{
-				t = pt.GetTypeArgument(0);
+				t = t.TypeArguments[0];
 			}
 			return t.GetDelegateInvokeMethod();
 		}
@@ -571,8 +570,9 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		{
 			Log.WriteLine("MakeExactInference from " + U + " to " + V);
 
-			if (V is NullabilityAnnotatedTypeParameter nullableTP) {
-				V = nullableTP.OriginalTypeParameter;
+			if (U.Nullability == V.Nullability) {
+				U = U.WithoutNullability();
+				V = V.WithoutNullability();
 			}
 
 			// If V is one of the unfixed Xi then U is added to the set of bounds for Xi.
@@ -613,8 +613,10 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		
 		TP GetTPForType(IType v)
 		{
-			ITypeParameter p = v as ITypeParameter;
-			if (p != null) {
+			if (v is NullabilityAnnotatedTypeParameter natp) {
+				v = natp.OriginalTypeParameter;
+			}
+			if (v is ITypeParameter p) {
 				int index = p.Index;
 				if (index < typeParameters.Length && typeParameters[index].TypeParameter == p)
 					return typeParameters[index];
@@ -631,7 +633,11 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		void MakeLowerBoundInference(IType U, IType V)
 		{
 			Log.WriteLine(" MakeLowerBoundInference from " + U + " to " + V);
-			
+			if (U.Nullability == V.Nullability) {
+				U = U.WithoutNullability();
+				V = V.WithoutNullability();
+			}
+
 			// If V is one of the unfixed Xi then U is added to the set of bounds for Xi.
 			TP tp = GetTPForType(V);
 			if (tp != null && tp.IsFixed == false) {
@@ -728,7 +734,11 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		void MakeUpperBoundInference(IType U, IType V)
 		{
 			Log.WriteLine(" MakeUpperBoundInference from " + U + " to " + V);
-			
+			if (U.Nullability == V.Nullability) {
+				U = U.WithoutNullability();
+				V = V.WithoutNullability();
+			}
+
 			// If V is one of the unfixed Xi then U is added to the set of bounds for Xi.
 			TP tp = GetTPForType(V);
 			if (tp != null && tp.IsFixed == false) {

ICSharpCode.Decompiler/TypeSystem/Implementation/NullabilityAnnotatedType.cs

@@ -14,7 +14,8 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		internal NullabilityAnnotatedType(IType type, Nullability nullability)
 			: base(type)
 		{
-			Debug.Assert(nullability != type.Nullability);
+			Debug.Assert(type.Nullability == Nullability.Oblivious);
+			Debug.Assert(nullability != Nullability.Oblivious);
 			// Due to IType -> concrete type casts all over the type system, we can insert
 			// the NullabilityAnnotatedType wrapper only in some limited places.
 			Debug.Assert(type is ITypeDefinition

ICSharpCode.Decompiler/TypeSystem/TupleType.cs

@@ -373,7 +373,8 @@ namespace ICSharpCode.Decompiler.TypeSystem
 	{
 		public static IType TupleUnderlyingTypeOrSelf(this IType type)
 		{
-			return (type as TupleType)?.UnderlyingType ?? type;
+			var t = (type as TupleType)?.UnderlyingType ?? type;
+			return t.WithoutNullability();
 		}
 	}
 }

ICSharpCode.Decompiler/TypeSystem/TypeSystemExtensions.cs

@@ -530,5 +530,10 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		{
 			return KnownAttributes.IsKnownAttributeType(type);
 		}
+
+		public static IType WithoutNullability(this IType type)
+		{
+			return type.ChangeNullability(Nullability.Oblivious);
+		}
 	}
 }