#2685: Emit 'override' without 'newslot' as 'virtual' if there is no (known) method to override.

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/FSharpLoops_Debug.cs

@@ -92,7 +92,7 @@ public static class Program
 			pc = 2;
 		}
 
-		public override bool get_CheckClose()
+		public bool get_CheckClose()
 		{
 			switch (pc)
 			{
@@ -106,7 +106,7 @@ public static class Program
 
 		[DebuggerNonUserCode]
 		[CompilerGenerated]
-		public override int get_LastGenerated()
+		public int get_LastGenerated()
 		{
 			return current;
 		}

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/FSharpLoops_Release.cs

@@ -93,7 +93,7 @@ public static class Program
 			pc = 2;
 		}
 
-		public override bool get_CheckClose()
+		public bool get_CheckClose()
 		{
 			switch (pc)
 			{
@@ -107,7 +107,7 @@ public static class Program
 
 		[DebuggerNonUserCode]
 		[CompilerGenerated]
-		public override int get_LastGenerated()
+		public int get_LastGenerated()
 		{
 			return current;
 		}

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/SequenceOfNestedIfs.cs

@@ -12,11 +12,11 @@ public class SequenceOfNestedIfs
 {
 	public bool _clear;
 	public Material _material;
-	public override bool CheckShader()
+	public virtual bool CheckShader()
 	{
 		return false;
 	}
-	public override void CreateMaterials()
+	public virtual void CreateMaterials()
 	{
 		if (!_clear)
 		{

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/UnknownTypes.cs

@@ -2,7 +2,7 @@ internal class UnknownTypes
 {
 	private readonly IInterface memberField;
 
-	public override bool CanExecute(CallbackQuery message)
+	public virtual bool CanExecute(CallbackQuery message)
 	{
 		return ((IInterface<SomeClass, bool>)(object)memberField).Execute(new SomeClass {
 			ChatId = StaticClass.GetChatId(message),

ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs

@@ -1570,14 +1570,16 @@ namespace ICSharpCode.Decompiler.CSharp
 				{
 					SetNewModifier(methodDecl);
 				}
-				//else if (!method.IsVirtual && method.IsOverride && InheritanceHelper.GetBaseMember(method) == null)
-				//{
-				//	methodDecl.Modifiers &= ~Modifiers.Override;
-				//	if (!method.DeclaringTypeDefinition.IsSealed)
-				//	{
-				//		methodDecl.Modifiers |= Modifiers.Virtual;
-				//	}
-				//}
+				else if (!method.IsStatic && !method.IsExplicitInterfaceImplementation
+					&& !method.IsVirtual && method.IsOverride
+					&& InheritanceHelper.GetBaseMember(method) == null && IsTypeHierarchyKnown(method.DeclaringType))
+				{
+					methodDecl.Modifiers &= ~Modifiers.Override;
+					if (!method.DeclaringTypeDefinition.IsSealed)
+					{
+						methodDecl.Modifiers |= Modifiers.Virtual;
+					}
+				}
 				if (IsCovariantReturnOverride(method))
 				{
 					RemoveAttribute(methodDecl, KnownAttribute.PreserveBaseOverrides);
@@ -1585,6 +1587,21 @@ namespace ICSharpCode.Decompiler.CSharp
 					methodDecl.Modifiers |= Modifiers.Override;
 				}
 				return methodDecl;
+
+				bool IsTypeHierarchyKnown(IType type)
+				{
+					var definition = type.GetDefinition();
+					if (definition == null)
+					{
+						return false;
+					}
+
+					if (decompileRun.TypeHierarchyIsKnown.TryGetValue(definition, out var value))
+						return value;
+					value = method.DeclaringType.GetNonInterfaceBaseTypes().All(t => t.Kind != TypeKind.Unknown);
+					decompileRun.TypeHierarchyIsKnown.Add(definition, value);
+					return value;
+				}
 			}
 			finally
 			{

ICSharpCode.Decompiler/DecompileRun.cs

@@ -19,6 +19,8 @@ namespace ICSharpCode.Decompiler
 		public IDocumentationProvider DocumentationProvider { get; set; }
 		public Dictionary<ITypeDefinition, RecordDecompiler> RecordDecompilers { get; } = new Dictionary<ITypeDefinition, RecordDecompiler>();
 
+		public Dictionary<ITypeDefinition, bool> TypeHierarchyIsKnown { get; } = new();
+
 		Lazy<CSharp.TypeSystem.UsingScope> usingScope =>
 			new Lazy<CSharp.TypeSystem.UsingScope>(() => CreateUsingScope(Namespaces));