Improve inlining of boxed values

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

@@ -163,6 +163,7 @@
     <Compile Include="TestCases\Pretty\Issue3571_B.cs" />
     <Compile Include="TestCases\Pretty\Issue3571_A.cs" />
     <Compile Include="TestCases\Pretty\Issue3576.cs" />
+    <Compile Include="TestCases\Pretty\Issue3584.cs" />
     <Compile Include="TestCases\Pretty\PlaystationPreferPrimary.cs" />
     <Compile Include="TestCases\Pretty\Playstation.cs" />
     <None Include="TestCases\Ugly\NoLocalFunctions.Expected.cs" />

ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs

@@ -718,6 +718,12 @@ namespace ICSharpCode.Decompiler.Tests
 			await RunForLibrary(cscOptions: cscOptions);
 		}
 
+		[Test]
+		public async Task Issue3584([ValueSource(nameof(defaultOptions))] CompilerOptions cscOptions)
+		{
+			await RunForLibrary(cscOptions: cscOptions);
+		}
+
 		[Test]
 		public async Task AssemblyCustomAttributes([ValueSource(nameof(defaultOptions))] CompilerOptions cscOptions)
 		{

ICSharpCode.Decompiler.Tests/TestCases/Pretty/Issue3584.cs

@@ -0,0 +1,47 @@
+using System.Collections;
+using System.Collections.Generic;
+
+namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
+{
+	internal abstract class Issue4000<T> : IEnumerable<T>, IEnumerable
+	{
+		public int Length;
+
+		protected T[] results;
+
+#if !ROSLYN4
+		public T this[int i] {
+			get {
+				if (i >= Length || i < 0)
+				{
+					return default(T);
+				}
+				if (results[i] != null && results[i].Equals(default(T)))
+				{
+					results[i] = CreateIthElement(i);
+				}
+				return results[i];
+			}
+		}
+#endif
+
+		protected abstract T CreateIthElement(int i);
+
+		public IEnumerator<T> GetEnumerator()
+		{
+			for (int i = 0; i < Length; i++)
+			{
+				if (results[i] != null && results[i].Equals(default(T)))
+				{
+					results[i] = CreateIthElement(i);
+				}
+				yield return results[i];
+			}
+		}
+
+		IEnumerator IEnumerable.GetEnumerator()
+		{
+			return GetEnumerator();
+		}
+	}
+}

ICSharpCode.Decompiler/IL/ILTypeExtensions.cs

@@ -238,6 +238,8 @@ namespace ICSharpCode.Decompiler.IL
 						default:
 							return SpecialType.UnknownType;
 					}
+				case DefaultValue defaultValue:
+					return defaultValue.Type;
 				case ILFunction func when func.DelegateType != null:
 					return func.DelegateType;
 				default:

ICSharpCode.Decompiler/IL/Instructions.cs

@@ -318,6 +318,7 @@ namespace ICSharpCode.Decompiler.IL
 			set {
 				ValidateChild(value);
 				SetChildInstruction(ref this.argument, value, 0);
+				InvalidateFlags();
 			}
 		}
 		protected sealed override int GetChildCount()
@@ -4357,10 +4358,19 @@ namespace ICSharpCode.Decompiler.IL
 		public override StackType ResultType { get { return StackType.O; } }
 		protected override InstructionFlags ComputeFlags()
 		{
-			return base.ComputeFlags() | InstructionFlags.SideEffect | InstructionFlags.MayThrow;
+			var baseFlags = base.ComputeFlags();
+			if (baseFlags == InstructionFlags.None && Type.Equals(Argument.InferType(null)))
+			{
+				return InstructionFlags.None;
+			}
+			return baseFlags | InstructionFlags.SideEffect | InstructionFlags.MayThrow;
 		}
 		public override InstructionFlags DirectFlags {
 			get {
+				if (Flags == InstructionFlags.None)
+				{
+					return InstructionFlags.None;
+				}
 				return base.DirectFlags | InstructionFlags.SideEffect | InstructionFlags.MayThrow;
 			}
 		}