Fix icsharpcode#2231: Add support for ldloca; dup; initobj pattern us…

…ed by Roslyn.

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

@@ -285,5 +285,15 @@ public static void Issue1959(int a, int b, int? c)
 			Console.WriteLine("{}, {}", a < b, a > b);
 			Console.WriteLine("{}, {}", a < Environment.GetLogicalDrives().Length, a > (c ?? b));
 		}
+
+		public static Type Issue2231<T>()
+		{
+			return default(T).GetType();
+		}
+
+		public static string Issue2231b<T>()
+		{
+			return default(T).ToString();
+		}
 	}
 }

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

@@ -278,5 +278,15 @@ public static string CallOnIntegerConstant()
 		{
 			return ulong.MaxValue.ToString();
 		}
+
+		public static void M4()
+		{
+			Test(default(DateTime).GetType());
+			Test(default(DateTime).ToString());
+		}
+
+		public static void Test(object x)
+		{
+		}
 	}
 }

ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs

@@ -87,6 +87,7 @@ public static List<IILTransform> GetILTransforms()
 				new AsyncAwaitDecompiler(),  // must run after inlining but before loop detection
 				new DetectCatchWhenConditionBlocks(), // must run after inlining but before loop detection
 				new DetectExitPoints(canIntroduceExitForReturn: false),
+				new LdLocaDupInitObjTransform(),
 				new EarlyExpressionTransforms(),
 				// RemoveDeadVariableInit must run after EarlyExpressionTransforms so that stobj(ldloca V, ...)
 				// is already collapsed into stloc(V, ...).

ICSharpCode.Decompiler/ICSharpCode.Decompiler.csproj

@@ -73,6 +73,7 @@
     <Compile Include="CSharp\TranslatedStatement.cs" />
     <Compile Include="IL\ControlFlow\AwaitInFinallyTransform.cs" />
     <Compile Include="IL\Transforms\IntroduceNativeIntTypeOnLocals.cs" />
+    <Compile Include="IL\Transforms\LdLocaDupInitObjTransform.cs" />
     <Compile Include="Semantics\OutVarResolveResult.cs" />
     <Compile Include="CSharp\Syntax\AstNode.cs" />
     <Compile Include="CSharp\Syntax\AstNodeCollection.cs" />

ICSharpCode.Decompiler/IL/Transforms/LdLocaDupInitObjTransform.cs

@@ -0,0 +1,77 @@
+// Copyright (c) 2021 Siegfried Pammer
+// 
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+// 
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+// 
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+#nullable enable
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+using ICSharpCode.Decompiler.TypeSystem;
+
+namespace ICSharpCode.Decompiler.IL.Transforms
+{
+	// stloc s(ldloca v)
+	// stobj System.DateTime(ldloc s, default.value T)
+	// where s.LoadCount > 1
+	// which is: ldloca; dup; initobj in IL (generated by Roslyn >= 2)
+	// =>
+	// stloc v(default.value T)
+	// stloc s(ldloca v)
+	// which is: ldloca; ldloca; initobj in IL (generated by legacy csc)
+	// 
+	// The second pattern allows inlining in the subsequent uses.
+	class LdLocaDupInitObjTransform : IILTransform
+	{
+		void IILTransform.Run(ILFunction function, ILTransformContext context)
+		{
+			foreach (var block in function.Descendants.OfType<Block>())
+			{
+				for (int i = 0; i < block.Instructions.Count; i++)
+				{
+					TryTransform(block, i, context);
+				}
+			}
+		}
+
+		private bool TryTransform(Block block, int i, ILTransformContext context)
+		{
+			if (block.Instructions[i] is not StLoc { Variable: var s, Value: LdLoca { Variable: var v } } inst1)
+			{
+				return false;
+			}
+			if (block.Instructions.ElementAtOrDefault(i + 1) is not StObj inst2)
+			{
+				return false;
+			}
+			if (!(inst2.Target.MatchLdLoc(s)
+				&& TypeUtils.IsCompatibleTypeForMemoryAccess(v.Type, inst2.Type)
+				&& inst2.UnalignedPrefix == 0
+				&& !inst2.IsVolatile
+				&& inst2.Value is DefaultValue))
+			{
+				return false;
+			}
+			context.Step("LdLocaDupInitObjTransform", inst1);
+			block.Instructions[i] = new StLoc(v, inst2.Value).WithILRange(inst2);
+			block.Instructions[i + 1] = inst1;
+			return true;
+		}
+	}
+}