Fix #1442: Allow "dynamic?" in the type system.

ICSharpCode.Decompiler.Tests/TestCases/Pretty/NullableRefTypes.cs

@@ -7,6 +7,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 	{
 		private string field_string;
 		private string? field_nullable_string;
+		private dynamic? field_nullable_dynamic;
 
 		private Dictionary<string?, string> field_generic;
 		private (string, string?, string) field_tuple;

ICSharpCode.Decompiler/TypeSystem/Implementation/DummyTypeParameter.cs

@@ -168,5 +168,14 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		bool ITypeParameter.HasValueTypeConstraint => false;
 		bool ITypeParameter.HasUnmanagedConstraint => false;
 		Nullability ITypeParameter.NullabilityConstraint => Nullability.Oblivious;
+
+		public override IType ChangeNullability(Nullability nullability)
+		{
+			if (nullability == Nullability.Oblivious) {
+				return this;
+			} else {
+				return new NullabilityAnnotatedTypeParameter(this, nullability);
+			}
+		}
 	}
 }

ICSharpCode.Decompiler/TypeSystem/Implementation/NullabilityAnnotatedType.cs

@@ -18,6 +18,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			// 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
+				|| type.Kind == TypeKind.Dynamic
 				|| (type is ITypeParameter && this is ITypeParameter));
 			this.nullability = nullability;
 		}

ICSharpCode.Decompiler/TypeSystem/SpecialType.cs

@@ -101,5 +101,13 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		{
 			return 81625621 ^ (int)kind;
 		}
+
+		public override IType ChangeNullability(Nullability nullability)
+		{
+			if (nullability == base.Nullability)
+				return this;
+			else
+				return new NullabilityAnnotatedType(this, nullability);
+		}
 	}
 }