Upgrade C# 8.0 nullability support for Roslyn 3.2.0-beta4.

ICSharpCode.Decompiler.Tests/Helpers/RemoveCompilerAttribute.cs

@@ -40,6 +40,7 @@ namespace ICSharpCode.Decompiler.Tests.Helpers
 			"System.Runtime.CompilerServices.IsByRefLikeAttribute",
 			"System.Runtime.CompilerServices.IsUnmanagedAttribute",
 			"System.Runtime.CompilerServices.NullableAttribute",
+			"System.Runtime.CompilerServices.NullableContextAttribute",
 			"Microsoft.CodeAnalysis.EmbeddedAttribute",
 		};
 

ICSharpCode.Decompiler.Tests/ICSharpCode.Decompiler.Tests.csproj

@@ -42,8 +42,8 @@
   <ItemGroup>
     <PackageReference Include="DiffLib" Version="2017.7.26.1241" />
     <PackageReference Include="Microsoft.Build.Locator" Version="1.2.2" />
-    <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="3.2.0-beta3-final" />
-    <PackageReference Include="Microsoft.CodeAnalysis.VisualBasic" Version="3.2.0-beta3-final" />
+    <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="3.2.0-beta4-final" />
+    <PackageReference Include="Microsoft.CodeAnalysis.VisualBasic" Version="3.2.0-beta4-final" />
     <PackageReference Include="Microsoft.DiaSymReader.Converter.Xml" Version="1.1.0-beta1-63314-01" />
     <PackageReference Include="NUnit3TestAdapter" Version="3.13.0" />
     <PackageReference Include="System.Collections.Immutable" Version="1.5.0" />

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

@@ -1,15 +1,20 @@
 #nullable enable
+using System;
 using System.Collections.Generic;
 
 namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 {
-	public class NullableRefTypes
+	public class T01_NullableRefTypes
 	{
 		private string field_string;
 		private string? field_nullable_string;
 		private dynamic? field_nullable_dynamic;
 
 		private Dictionary<string?, string> field_generic;
+		private Dictionary<int, string?[]> field_generic2;
+		private Dictionary<int?, string?[]> field_generic3;
+		private KeyValuePair<string?, string> field_generic_value_type;
+		private KeyValuePair<string?, string>? field_generic_nullable_value_type;
 		private (string, string?, string) field_tuple;
 		private string[]?[] field_array;
 		private Dictionary<(string, string?), (int, string[]?, string?[])> field_complex;
@@ -28,5 +33,50 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		{
 			return field_nullable_string?.Length + arr?.Length;
 		}
+
+		public void GenericNullable<T1, T2>((T1?, T1, T2, T2?, T1, T1?) x) where T1 : class where T2 : struct
+		{
+		}
+
+		public T ByRef<T>(ref T t)
+		{
+			return t;
+		}
+
+		public void CallByRef(ref string a, ref string? b)
+		{
+			ByRef(ref a).ToString();
+			ByRef(ref b)!.ToString();
+		}
+	}
+
+	public class T02_EverythingIsNullableInHere
+	{
+		private string? field1;
+		private object? field2;
+		// value types are irrelevant for the nullability attributes:
+		private int field3;
+		private int? field4;
+
+		public string? Property {
+			get;
+			set;
+		}
+		public event EventHandler? Event;
+	}
+
+	public class T03_EverythingIsNotNullableInHere
+	{
+		private string field1;
+		private object field2;
+		// value types are irrelevant for the nullability attributes:
+		private int field3;
+		private int? field4;
+
+		public string Property {
+			get;
+			set;
+		}
+		public event EventHandler Event;
 	}
 }

ICSharpCode.Decompiler/CSharp/Resolver/Log.cs

@@ -29,7 +29,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 	/// </summary>
 	static class Log
 	{
-		const bool logEnabled = false;
+		const bool logEnabled = true;
 #if __MonoCS__
 		[Conditional("MCS_DEBUG")]
 #else

ICSharpCode.Decompiler/CSharp/Resolver/TypeInference.cs

@@ -570,7 +570,11 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		void MakeExactInference(IType U, IType V)
 		{
 			Log.WriteLine("MakeExactInference from " + U + " to " + V);
-			
+
+			if (V is NullabilityAnnotatedTypeParameter nullableTP) {
+				V = nullableTP.OriginalTypeParameter;
+			}
+
 			// 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/CSharp/Syntax/ComposedType.cs

@@ -83,6 +83,12 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			}
 		}
 
+		public bool HasOnlyNullableSpecifier {
+			get {
+				return HasNullableSpecifier && !HasRefSpecifier && !HasReadOnlySpecifier && PointerRank == 0 && ArraySpecifiers.Count == 0;
+			}
+		}
+
 		public CSharpTokenNode NullableSpecifierToken {
 			get {
 				return GetChildByRole(NullableRole);

ICSharpCode.Decompiler/CSharp/Transforms/PatternStatementTransform.cs

@@ -777,8 +777,16 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				default:
 					return false;
 			}
-			if (!ev.ReturnType.IsMatch(m.Get("type").Single()))
-				return false; // variable types must match event type
+			if (!ev.ReturnType.IsMatch(m.Get("type").Single())) {
+				// Variable types must match event type,
+				// except that the event type may have an additional nullability annotation
+				if (ev.ReturnType is ComposedType ct && ct.HasOnlyNullableSpecifier) {
+					if (!ct.BaseType.IsMatch(m.Get("type").Single()))
+						return false;
+				} else {
+					return false;
+				}
+			}
 			var combineMethod = m.Get<AstNode>("delegateCombine").Single().Parent.GetSymbol() as IMethod;
 			if (combineMethod == null || combineMethod.Name != (isAddAccessor ? "Combine" : "Remove"))
 				return false;

ICSharpCode.Decompiler/CSharp/TranslatedExpression.cs

@@ -413,8 +413,15 @@ namespace ICSharpCode.Decompiler.CSharp
 					.WithILInstruction(this.ILInstructions)
 					.WithRR(new ConstantResolveResult(targetType, null));
 			}
-			if (allowImplicitConversion && conversions.ImplicitConversion(ResolveResult, targetType).IsValid) {
-				return this;
+			if (allowImplicitConversion) {
+				if (conversions.ImplicitConversion(ResolveResult, targetType).IsValid) {
+					return this;
+				}
+			} else {
+				if (NormalizeTypeVisitor.RemoveModifiersAndNullability.EquivalentTypes(type, targetType)) {
+					// avoid an explicit cast when types differ only in nullability of reference types
+					return this;
+				}
 			}
 			var castExpr = new CastExpression(expressionBuilder.ConvertType(targetType), Expression);
 			bool needsCheckAnnotation = targetUType.GetStackType().IsIntegerType();

ICSharpCode.Decompiler/SRMExtensions.cs

@@ -1,13 +1,10 @@
 using System;
-using System.Collections.Generic;
 using System.Collections.Immutable;
-using System.Linq;
 using System.Reflection;
 using System.Reflection.Metadata;
 using SRM = System.Reflection.Metadata;
 using System.Reflection.PortableExecutable;
 using ICSharpCode.Decompiler.TypeSystem;
-using ICSharpCode.Decompiler.TypeSystem.Implementation;
 using ICSharpCode.Decompiler.Util;
 using System.Reflection.Metadata.Ecma335;
 
@@ -371,6 +368,28 @@ namespace ICSharpCode.Decompiler
 		{
 			return attr.GetAttributeType(metadata).IsKnownType(metadata, attrType);
 		}
+
+		public static Nullability? GetNullableContext(this CustomAttributeHandleCollection customAttributes, MetadataReader metadata)
+		{
+			foreach (var handle in customAttributes) {
+				var customAttribute = metadata.GetCustomAttribute(handle);
+				if (customAttribute.IsKnownAttribute(metadata, KnownAttribute.NullableContext)) {
+					// Decode 
+					CustomAttributeValue<IType> value;
+					try {
+						value = customAttribute.DecodeValue(Metadata.MetadataExtensions.MinimalAttributeTypeProvider);
+					} catch (BadImageFormatException) {
+						continue;
+					} catch (Metadata.EnumUnderlyingTypeResolveException) {
+						continue;
+					}
+					if (value.FixedArguments.Length == 1 && value.FixedArguments[0].Value is byte b && b <= 2) {
+						return (Nullability)b;
+					}
+				}
+			}
+			return null;
+		}
 		#endregion
 
 		public static unsafe SRM.BlobReader GetInitialValue(this FieldDefinition field, PEReader pefile, ICompilation typeSystem)

ICSharpCode.Decompiler/TypeSystem/ApplyAttributeTypeVisitor.cs

@@ -37,14 +37,19 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			SRM.CustomAttributeHandleCollection? attributes,
 			SRM.MetadataReader metadata,
 			TypeSystemOptions options,
+			Nullability nullableContext,
 			bool typeChildrenOnly = false)
 		{
 			bool hasDynamicAttribute = false;
 			bool[] dynamicAttributeData = null;
 			string[] tupleElementNames = null;
-			bool hasNullableAttribute = false;
-			Nullability nullability = Nullability.Oblivious;
+			Nullability nullability;
 			Nullability[] nullableAttributeData = null;
+			if ((options & TypeSystemOptions.NullabilityAnnotations) != 0) {
+				nullability = nullableContext;
+			} else {
+				nullability = Nullability.Oblivious;
+			}
 			const TypeSystemOptions relevantOptions = TypeSystemOptions.Dynamic | TypeSystemOptions.Tuple | TypeSystemOptions.NullabilityAnnotations;
 			if (attributes != null && (options & relevantOptions) != 0) {
 				foreach (var attrHandle in attributes.Value) {
@@ -70,7 +75,6 @@ namespace ICSharpCode.Decompiler.TypeSystem
 							}
 						}
 					} else if ((options & TypeSystemOptions.NullabilityAnnotations) != 0 && attrType.IsKnownType(metadata, KnownAttribute.Nullable)) {
-						hasNullableAttribute = true;
 						var ctor = attr.DecodeValue(Metadata.MetadataExtensions.minimalCorlibTypeProvider);
 						if (ctor.FixedArguments.Length == 1) {
 							var arg = ctor.FixedArguments[0];
@@ -78,13 +82,15 @@ namespace ICSharpCode.Decompiler.TypeSystem
 								&& values.All(v => v.Value is byte b && b <= 2)) {
 								nullableAttributeData = values.SelectArray(v => (Nullability)(byte)v.Value);
 							} else if (arg.Value is byte b && b <= 2) {
-								nullability = (Nullability)(byte)arg.Value;
+								nullability = (Nullability)b;
 							}
 						}
 					}
 				}
 			}
-			if (hasDynamicAttribute || hasNullableAttribute || (options & (TypeSystemOptions.Tuple | TypeSystemOptions.KeepModifiers)) != TypeSystemOptions.KeepModifiers) {
+			if (hasDynamicAttribute || nullability != Nullability.Oblivious || nullableAttributeData != null 
+				|| (options & (TypeSystemOptions.Tuple | TypeSystemOptions.KeepModifiers)) != TypeSystemOptions.KeepModifiers)
+			{
 				var visitor = new ApplyAttributeTypeVisitor(
 					compilation, hasDynamicAttribute, dynamicAttributeData,
 					options, tupleElementNames,
@@ -148,16 +154,21 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		Nullability GetNullability()
 		{
 			if (nullabilityTypeIndex < nullableAttributeData?.Length)
-				return nullableAttributeData[nullabilityTypeIndex];
+				return nullableAttributeData[nullabilityTypeIndex++];
 			else
 				return defaultNullability;
 		}
 
+		void ExpectDummyNullabilityForGenericValueType()
+		{
+			var n = GetNullability();
+			Debug.Assert(n == Nullability.Oblivious);
+		}
+
 		public override IType VisitArrayType(ArrayType type)
 		{
 			var nullability = GetNullability();
 			dynamicTypeIndex++;
-			nullabilityTypeIndex++;
 			return base.VisitArrayType(type).ChangeNullability(nullability);
 		}
 
@@ -181,18 +192,18 @@ namespace ICSharpCode.Decompiler.TypeSystem
 						elementNames = ImmutableArray.CreateRange(extractedValues);
 					}
 					tupleTypeIndex += tupleCardinality;
+					ExpectDummyNullabilityForGenericValueType();
 					var elementTypes = ImmutableArray.CreateBuilder<IType>(tupleCardinality);
 					do {
 						int normalArgCount = Math.Min(type.TypeArguments.Count, TupleType.RestPosition - 1);
 						for (int i = 0; i < normalArgCount; i++) {
 							dynamicTypeIndex++;
-							nullabilityTypeIndex++;
 							elementTypes.Add(type.TypeArguments[i].AcceptVisitor(this));
 						}
 						if (type.TypeArguments.Count == TupleType.RestPosition) {
 							type = type.TypeArguments.Last() as ParameterizedType;
+							ExpectDummyNullabilityForGenericValueType();
 							dynamicTypeIndex++;
-							nullabilityTypeIndex++;
 							if (type != null && TupleType.IsTupleCompatible(type, out int nestedCardinality)) {
 								tupleTypeIndex += nestedCardinality;
 							} else {
@@ -218,11 +229,13 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			// Visit generic type and type arguments.
 			// Like base implementation, except that it increments dynamicTypeIndex.
 			var genericType = type.GenericType.AcceptVisitor(this);
+			if (genericType.IsReferenceType != true && !genericType.IsKnownType(KnownTypeCode.NullableOfT)) {
+				ExpectDummyNullabilityForGenericValueType();
+			}
 			bool changed = type.GenericType != genericType;
 			var arguments = new IType[type.TypeArguments.Count];
 			for (int i = 0; i < type.TypeArguments.Count; i++) {
 				dynamicTypeIndex++;
-				nullabilityTypeIndex++;
 				arguments[i] = type.TypeArguments[i].AcceptVisitor(this);
 				changed = changed || arguments[i] != type.TypeArguments[i];
 			}
@@ -240,8 +253,28 @@ namespace ICSharpCode.Decompiler.TypeSystem
 				else if (dynamicAttributeData[dynamicTypeIndex])
 					newType = SpecialType.Dynamic;
 			}
+			if (type.IsReferenceType == true) {
+				Nullability nullability = GetNullability();
+				return newType.ChangeNullability(nullability);
+			} else {
+				return newType;
+			}
+		}
+
+		public override IType VisitOtherType(IType type)
+		{
+			type = base.VisitOtherType(type);
+			if (type.Kind == TypeKind.Unknown && type.IsReferenceType == true) {
+				Nullability nullability = GetNullability();
+				type = type.ChangeNullability(nullability);
+			}
+			return type;
+		}
+
+		public override IType VisitTypeParameter(ITypeParameter type)
+		{
 			Nullability nullability = GetNullability();
-			return newType.ChangeNullability(nullability);
+			return type.ChangeNullability(nullability);
 		}
 	}
 }

ICSharpCode.Decompiler/TypeSystem/ITypeDefinition.cs

@@ -67,5 +67,11 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// </summary>
 		/// <remarks>This property is used to speed up the search for extension methods.</remarks>
 		bool HasExtensionMethods { get; }
+
+		/// <summary>
+		/// The nullability specified in the [NullableContext] attribute on the type.
+		/// This serves as default nullability for members of the type that do not have a [Nullable] attribute.
+		/// </summary>
+		Nullability NullableContext { get; }
 	}
 }

ICSharpCode.Decompiler/TypeSystem/Implementation/AttributeListBuilder.cs

@@ -208,6 +208,8 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 							return (options & TypeSystemOptions.UnmanagedConstraints) != 0 && target == SymbolKind.TypeParameter;
 						case "NullableAttribute":
 							return (options & TypeSystemOptions.NullabilityAnnotations) != 0;
+						case "NullableContextAttribute":
+							return (options & TypeSystemOptions.NullabilityAnnotations) != 0 && (target == SymbolKind.TypeDefinition || target == SymbolKind.Method);
 						default:
 							return false;
 					}

ICSharpCode.Decompiler/TypeSystem/Implementation/KnownAttributes.cs

@@ -40,6 +40,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		Dynamic,
 		TupleElementNames,
 		Nullable,
+		NullableContext,
 		Conditional,
 		Obsolete,
 		IsReadOnly,
@@ -107,6 +108,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			new TopLevelTypeName("System.Runtime.CompilerServices", nameof(DynamicAttribute)),
 			new TopLevelTypeName("System.Runtime.CompilerServices", nameof(TupleElementNamesAttribute)),
 			new TopLevelTypeName("System.Runtime.CompilerServices", "NullableAttribute"),
+			new TopLevelTypeName("System.Runtime.CompilerServices", "NullableContextAttribute"),
 			new TopLevelTypeName("System.Diagnostics", nameof(ConditionalAttribute)),
 			new TopLevelTypeName("System", nameof(ObsoleteAttribute)),
 			new TopLevelTypeName("System.Runtime.CompilerServices", "IsReadOnlyAttribute"),

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataEvent.cs

@@ -79,8 +79,10 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 					return returnType;
 				var metadata = module.metadata;
 				var ev = metadata.GetEventDefinition(handle);
-				var context = new GenericContext(DeclaringTypeDefinition?.TypeParameters);
-				returnType = module.ResolveType(ev.Type, context, ev.GetCustomAttributes());
+				var declaringTypeDef = DeclaringTypeDefinition;
+				var context = new GenericContext(declaringTypeDef?.TypeParameters);
+				var nullableContext = declaringTypeDef?.NullableContext ?? Nullability.Oblivious;
+				returnType = module.ResolveType(ev.Type, context, ev.GetCustomAttributes(), nullableContext);
 				return LazyInit.GetOrSet(ref this.returnType, returnType);
 			}
 		}

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataField.cs

@@ -186,7 +186,8 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 					ty = mod.ElementType;
 				}
 				ty = ApplyAttributeTypeVisitor.ApplyAttributesToType(ty, Compilation,
-					fieldDef.GetCustomAttributes(), metadata, module.TypeSystemOptions);
+					fieldDef.GetCustomAttributes(), metadata, module.TypeSystemOptions,
+					DeclaringTypeDefinition?.NullableContext ?? Nullability.Oblivious);
 			} catch (BadImageFormatException) {
 				ty = SpecialType.UnknownType;
 			}

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataMethod.cs

@@ -149,6 +149,13 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			}
 		}
 
+		internal Nullability NullableContext {
+			get {
+				var methodDef = module.metadata.GetMethodDefinition(handle);
+				return methodDef.GetCustomAttributes().GetNullableContext(module.metadata) ?? DeclaringTypeDefinition.NullableContext;
+			}
+		}
+
 		private void DecodeSignature()
 		{
 			var methodDef = module.metadata.GetMethodDefinition(handle);
@@ -156,8 +163,9 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			IType returnType;
 			IParameter[] parameters;
 			try {
+				var nullableContext = methodDef.GetCustomAttributes().GetNullableContext(module.metadata) ?? DeclaringTypeDefinition.NullableContext;
 				var signature = methodDef.DecodeSignature(module.TypeProvider, genericContext);
-				(returnType, parameters) = DecodeSignature(module, this, signature, methodDef.GetParameters());
+				(returnType, parameters) = DecodeSignature(module, this, signature, methodDef.GetParameters(), nullableContext);
 			} catch (BadImageFormatException) {
 				returnType = SpecialType.UnknownType;
 				parameters = Empty<IParameter>.Array;
@@ -166,7 +174,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			LazyInit.GetOrSet(ref this.parameters, parameters);
 		}
 
-		internal static (IType, IParameter[]) DecodeSignature(MetadataModule module, IParameterizedMember owner, MethodSignature<IType> signature, ParameterHandleCollection? parameterHandles)
+		internal static (IType, IParameter[]) DecodeSignature(MetadataModule module, IParameterizedMember owner, MethodSignature<IType> signature, ParameterHandleCollection? parameterHandles, Nullability nullableContext)
 		{
 			var metadata = module.metadata;
 			int i = 0;
@@ -187,14 +195,14 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 						// Fill gaps in the sequence with non-metadata parameters:
 						while (i < par.SequenceNumber - 1) {
 							parameterType = ApplyAttributeTypeVisitor.ApplyAttributesToType(
-								signature.ParameterTypes[i], module.Compilation, null, metadata, module.TypeSystemOptions);
+								signature.ParameterTypes[i], module.Compilation, null, metadata, module.TypeSystemOptions, nullableContext);
 							parameters[i] = new DefaultParameter(parameterType, name: string.Empty, owner,
 								isRef: parameterType.Kind == TypeKind.ByReference);
 							i++;
 						}
 						parameterType = ApplyAttributeTypeVisitor.ApplyAttributesToType(
 							signature.ParameterTypes[i], module.Compilation,
-							par.GetCustomAttributes(), metadata, module.TypeSystemOptions);
+							par.GetCustomAttributes(), metadata, module.TypeSystemOptions, nullableContext);
 						parameters[i] = new MetadataParameter(module, owner, parameterType, parameterHandle);
 						i++;
 					}
@@ -202,7 +210,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			}
 			while (i < signature.RequiredParameterCount) {
 				parameterType = ApplyAttributeTypeVisitor.ApplyAttributesToType(
-					signature.ParameterTypes[i], module.Compilation, null, metadata, module.TypeSystemOptions);
+					signature.ParameterTypes[i], module.Compilation, null, metadata, module.TypeSystemOptions, nullableContext);
 				parameters[i] = new DefaultParameter(parameterType, name: string.Empty, owner,
 					isRef: parameterType.Kind == TypeKind.ByReference);
 				i++;
@@ -213,7 +221,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			}
 			Debug.Assert(i == parameters.Length);
 			var returnType = ApplyAttributeTypeVisitor.ApplyAttributesToType(signature.ReturnType,
-				module.Compilation, returnTypeAttributes, metadata, module.TypeSystemOptions);
+				module.Compilation, returnTypeAttributes, metadata, module.TypeSystemOptions, nullableContext);
 			return (returnType, parameters);
 		}
 		#endregion

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataProperty.cs

@@ -123,13 +123,22 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 				var signature = propertyDef.DecodeSignature(module.TypeProvider, genericContext);
 				var accessors = propertyDef.GetAccessors();
 				ParameterHandleCollection? parameterHandles;
-				if (!accessors.Getter.IsNil)
-					parameterHandles = module.metadata.GetMethodDefinition(accessors.Getter).GetParameters();
-				else if (!accessors.Setter.IsNil)
-					parameterHandles = module.metadata.GetMethodDefinition(accessors.Setter).GetParameters();
-				else
+				Nullability nullableContext;
+				if (!accessors.Getter.IsNil) {
+					var getter = module.metadata.GetMethodDefinition(accessors.Getter);
+					parameterHandles = getter.GetParameters();
+					nullableContext = getter.GetCustomAttributes().GetNullableContext(module.metadata)
+						?? DeclaringTypeDefinition?.NullableContext ?? Nullability.Oblivious;
+				} else if (!accessors.Setter.IsNil) {
+					var setter = module.metadata.GetMethodDefinition(accessors.Setter);
+					parameterHandles = setter.GetParameters();
+					nullableContext = setter.GetCustomAttributes().GetNullableContext(module.metadata)
+						?? DeclaringTypeDefinition?.NullableContext ?? Nullability.Oblivious;
+				} else {
 					parameterHandles = null;
-				(returnType, parameters) = MetadataMethod.DecodeSignature(module, this, signature, parameterHandles);
+					nullableContext = DeclaringTypeDefinition?.NullableContext ?? Nullability.Oblivious;
+				}
+				(returnType, parameters) = MetadataMethod.DecodeSignature(module, this, signature, parameterHandles, nullableContext);
 			} catch (BadImageFormatException) {
 				returnType = SpecialType.UnknownType;
 				parameters = Empty<IParameter>.Array;

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataTypeDefinition.cs

@@ -51,6 +51,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		public KnownTypeCode KnownTypeCode { get; }
 		public IType EnumUnderlyingType { get; }
 		public bool HasExtensionMethods { get; }
+		public Nullability NullableContext { get; }
 
 		// lazy-loaded:
 		IMember[] members;
@@ -77,10 +78,14 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 				
 				// Create type parameters:
 				this.TypeParameters = MetadataTypeParameter.Create(module, this.DeclaringTypeDefinition, this, td.GetGenericParameters());
+
+				this.NullableContext = td.GetCustomAttributes().GetNullableContext(metadata) ?? this.DeclaringTypeDefinition.NullableContext;
 			} else {
 				// Create type parameters:
 				this.TypeParameters = MetadataTypeParameter.Create(module, this, td.GetGenericParameters());
 
+				this.NullableContext = td.GetCustomAttributes().GetNullableContext(metadata) ?? module.NullableContext;
+
 				var topLevelTypeName = fullTypeName.TopLevelTypeName;
 				for (int i = 0; i < KnownTypeReference.KnownTypeCodeCount; i++) {
 					var ktr = KnownTypeReference.Get((KnownTypeCode)i);
@@ -301,7 +306,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 				}
 				foreach (var h in interfaceImplCollection) {
 					var iface = metadata.GetInterfaceImplementation(h);
-					baseTypes.Add(module.ResolveType(iface.Interface, context, iface.GetCustomAttributes()));
+					baseTypes.Add(module.ResolveType(iface.Interface, context, iface.GetCustomAttributes(), Nullability.Oblivious));
 				}
 				return LazyInit.GetOrSet(ref this.directBaseTypes, baseTypes);
 			}

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataTypeParameter.cs

@@ -177,13 +177,21 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		{
 			var metadata = module.metadata;
 			var gp = metadata.GetGenericParameter(handle);
+			Nullability nullableContext;
+			if (Owner is ITypeDefinition typeDef) {
+				nullableContext = typeDef.NullableContext;
+			} else if (Owner is MetadataMethod method) {
+				nullableContext = method.NullableContext;
+			} else {
+				nullableContext = Nullability.Oblivious;
+			}
 
 			var constraintHandleCollection = gp.GetConstraints();
 			List<IType> result = new List<IType>(constraintHandleCollection.Count + 1);
 			bool hasNonInterfaceConstraint = false;
 			foreach (var constraintHandle in constraintHandleCollection) {
 				var constraint = metadata.GetGenericParameterConstraint(constraintHandle);
-				var ty = module.ResolveType(constraint.Type, new GenericContext(Owner), constraint.GetCustomAttributes());
+				var ty = module.ResolveType(constraint.Type, new GenericContext(Owner), constraint.GetCustomAttributes(), nullableContext);
 				result.Add(ty);
 				hasNonInterfaceConstraint |= (ty.Kind != TypeKind.Interface);
 			}

ICSharpCode.Decompiler/TypeSystem/Implementation/MinimalCorlib.cs

@@ -177,6 +177,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 
 			bool IType.IsByRefLike => false;
 			Nullability IType.Nullability => Nullability.Oblivious;
+			Nullability ITypeDefinition.NullableContext => Nullability.Oblivious;
 
 			IType IType.ChangeNullability(Nullability nullability)
 			{

ICSharpCode.Decompiler/TypeSystem/Implementation/NullabilityAnnotatedType.cs

@@ -19,6 +19,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			// the NullabilityAnnotatedType wrapper only in some limited places.
 			Debug.Assert(type is ITypeDefinition
 				|| type.Kind == TypeKind.Dynamic
+				|| type.Kind == TypeKind.Unknown
 				|| (type is ITypeParameter && this is ITypeParameter));
 			this.nullability = nullability;
 		}
@@ -50,10 +51,16 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		public override IType VisitChildren(TypeVisitor visitor)
 		{
 			IType newBase = baseType.AcceptVisitor(visitor);
-			if (newBase != baseType)
+			if (newBase != baseType) {
+				if (newBase.Nullability == Nullability.Nullable) {
+					// T?! -> T?
+					// This happens during type substitution for generic methods.
+					return newBase;
+				}
 				return newBase.ChangeNullability(nullability);
-			else
+			} else {
 				return this;
+			}
 		}
 
 		public override string ToString()
@@ -80,6 +87,8 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			this.baseType = type;
 		}
 
+		public ITypeParameter OriginalTypeParameter => baseType;
+
 		SymbolKind ITypeParameter.OwnerType => baseType.OwnerType;
 		IEntity ITypeParameter.Owner => baseType.Owner;
 		int ITypeParameter.Index => baseType.Index;

ICSharpCode.Decompiler/TypeSystem/Implementation/SpecializedMethod.cs

@@ -203,7 +203,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 				b.Append('[');
 				for (int i = 0; i < this.TypeArguments.Count; i++) {
 					if (i > 0) b.Append(", ");
-					b.Append(this.TypeArguments[i].ReflectionName);
+					b.Append(this.TypeArguments[i].ToString());
 				}
 				b.Append(']');
 			} else if (this.TypeParameters.Count > 0) {
@@ -216,7 +216,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 				b.Append(this.Parameters[i].ToString());
 			}
 			b.Append("):");
-			b.Append(this.ReturnType.ReflectionName);
+			b.Append(this.ReturnType.ToString());
 			b.Append(']');
 			return b.ToString();
 		}

ICSharpCode.Decompiler/TypeSystem/Implementation/TypeWithElementType.cs

@@ -46,7 +46,12 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		public override string ReflectionName {
 			get { return elementType.ReflectionName + NameSuffix; }
 		}
-		
+
+		public override string ToString()
+		{
+			return elementType.ToString() + NameSuffix;
+		}
+
 		public abstract string NameSuffix { get; }
 		
 		public IType ElementType {

ICSharpCode.Decompiler/TypeSystem/Implementation/UnknownType.cs

@@ -94,7 +94,15 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		public override bool? IsReferenceType {
 			get { return isReferenceType; }
 		}
-		
+
+		public override IType ChangeNullability(Nullability nullability)
+		{
+			if (nullability == Nullability.Oblivious)
+				return this;
+			else
+				return new NullabilityAnnotatedType(this, nullability);
+		}
+
 		public override int GetHashCode()
 		{
 			return (namespaceKnown ? 812571 : 12651) ^ fullTypeName.GetHashCode();

ICSharpCode.Decompiler/TypeSystem/MetadataModule.cs

@@ -40,6 +40,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		internal readonly MetadataReader metadata;
 		readonly TypeSystemOptions options;
 		internal readonly TypeProvider TypeProvider;
+		internal readonly Nullability NullableContext;
 
 		readonly MetadataNamespace rootNamespace;
 		readonly MetadataTypeDefinition[] typeDefs;
@@ -66,6 +67,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 				this.AssemblyName = metadata.GetString(moddef.Name);
 				this.FullAssemblyName = this.AssemblyName;
 			}
+			this.NullableContext = metadata.GetModuleDefinition().GetCustomAttributes().GetNullableContext(metadata) ?? Nullability.Oblivious;
 			this.rootNamespace = new MetadataNamespace(this, null, string.Empty, metadata.GetNamespaceDefinitionRoot());
 
 			if (!options.HasFlag(TypeSystemOptions.Uncached)) {
@@ -267,12 +269,12 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		#endregion
 
 		#region Resolve Type
-		public IType ResolveType(EntityHandle typeRefDefSpec, GenericContext context, CustomAttributeHandleCollection? typeAttributes = null)
+		public IType ResolveType(EntityHandle typeRefDefSpec, GenericContext context, CustomAttributeHandleCollection? typeAttributes = null, Nullability nullableContext = Nullability.Oblivious)
 		{
-			return ResolveType(typeRefDefSpec, context, options, typeAttributes);
+			return ResolveType(typeRefDefSpec, context, options, typeAttributes, nullableContext);
 		}
 
-		public IType ResolveType(EntityHandle typeRefDefSpec, GenericContext context, TypeSystemOptions customOptions,  CustomAttributeHandleCollection? typeAttributes = null)
+		public IType ResolveType(EntityHandle typeRefDefSpec, GenericContext context, TypeSystemOptions customOptions, CustomAttributeHandleCollection? typeAttributes = null, Nullability nullableContext = Nullability.Oblivious)
 		{
 			if (typeRefDefSpec.IsNil)
 				return SpecialType.UnknownType;
@@ -293,7 +295,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 				default:
 					throw new BadImageFormatException("Not a type handle");
 			}
-			ty = ApplyAttributeTypeVisitor.ApplyAttributesToType(ty, Compilation, typeAttributes, metadata, customOptions);
+			ty = ApplyAttributeTypeVisitor.ApplyAttributesToType(ty, Compilation, typeAttributes, metadata, customOptions, nullableContext);
 			return ty;
 		}
 
@@ -303,14 +305,14 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			var ty = ResolveType(declaringTypeReference, context,
 				options & ~(TypeSystemOptions.Dynamic | TypeSystemOptions.Tuple | TypeSystemOptions.NullabilityAnnotations));
 			// but substitute tuple types in type arguments:
-			ty = ApplyAttributeTypeVisitor.ApplyAttributesToType(ty, Compilation, null, metadata, options, typeChildrenOnly: true);
+			ty = ApplyAttributeTypeVisitor.ApplyAttributesToType(ty, Compilation, null, metadata, options, Nullability.Oblivious, typeChildrenOnly: true);
 			return ty;
 		}
 
 		IType IntroduceTupleTypes(IType ty)
 		{
 			// run ApplyAttributeTypeVisitor without attributes, in order to introduce tuple types
-			return ApplyAttributeTypeVisitor.ApplyAttributesToType(ty, Compilation, null, metadata, options);
+			return ApplyAttributeTypeVisitor.ApplyAttributesToType(ty, Compilation, null, metadata, options, Nullability.Oblivious);
 		}
 		#endregion
 

ICSharpCode.Decompiler/TypeSystem/NormalizeTypeVisitor.cs

@@ -21,6 +21,16 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			RemoveNullability = true,
 		};
 
+		internal static readonly NormalizeTypeVisitor RemoveModifiersAndNullability = new NormalizeTypeVisitor {
+			ReplaceClassTypeParametersWithDummy = false,
+			ReplaceMethodTypeParametersWithDummy = false,
+			DynamicAndObject = false,
+			TupleToUnderlyingType = false,
+			RemoveModOpt = true,
+			RemoveModReq = true,
+			RemoveNullability = true,
+		};
+
 		public bool EquivalentTypes(IType a, IType b)
 		{
 			a = a.AcceptVisitor(this);