From 442f6f90b7b362b4a6cc35ac386ecde77c83b2ad Mon Sep 17 00:00:00 2001
From: Siegfried Pammer <siegfriedpammer@gmail.com>
Date: Sun, 16 Aug 2020 11:49:39 +0200
Subject: [PATCH] Implement deconstruction discards

---
 .../TestCases/Pretty/DeconstructionTests.cs   | 57 +++++++++++++++++++
 .../IL/Instructions/DeconstructInstruction.cs |  5 +-
 .../IL/Transforms/DeconstructionTransform.cs  |  7 ++-
 3 files changed, 66 insertions(+), 3 deletions(-)

diff --git a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/DeconstructionTests.cs b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/DeconstructionTests.cs
index b68aa524a..e86156723 100644
--- a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/DeconstructionTests.cs
+++ b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/DeconstructionTests.cs
@@ -51,6 +51,21 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			}
 		}
 
+		private class DeconstructionSource<T, T2, T3>
+		{
+			public int Dummy {
+				get;
+				set;
+			}
+
+			public void Deconstruct(out T a, out T2 b, out T3 c)
+			{
+				a = default(T);
+				b = default(T2);
+				c = default(T3);
+			}
+		}
+
 		private class AssignmentTargets
 		{
 			public int IntField;
@@ -97,6 +112,11 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			return null;
 		}
 
+		private DeconstructionSource<T, T2, T3> GetSource<T, T2, T3>()
+		{
+			return null;
+		}
+
 		private AssignmentTargets Get(int i)
 		{
 			return null;
@@ -111,6 +131,33 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			Console.WriteLine(x);
 		}
 
+		public void LocalVariable_NoConversion_DiscardFirst()
+		{
+			MyInt x;
+			int value;
+			(_, x, value) = GetSource<MyInt?, MyInt, int>();
+			Console.WriteLine(x);
+			Console.WriteLine(value);
+		}
+
+		public void LocalVariable_NoConversion_DiscardLast()
+		{
+			MyInt? myInt3;
+			MyInt x;
+			(myInt3, x, _) = GetSource<MyInt?, MyInt, int>();
+			Console.WriteLine(myInt3);
+			Console.WriteLine(x);
+		}
+
+		public void LocalVariable_NoConversion_DiscardSecond()
+		{
+			MyInt? myInt3;
+			int value;
+			(myInt3, _, value) = GetSource<MyInt?, MyInt, int>();
+			Console.WriteLine(myInt3);
+			Console.WriteLine(value);
+		}
+
 		public void LocalVariable_NoConversion_ReferenceTypes()
 		{
 			string value;
@@ -144,5 +191,15 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		{
 			(Get(0).NMy, Get(1).My) = GetSource<MyInt?, MyInt>();
 		}
+
+		public void Property_NoConversion_DiscardFirst()
+		{
+			(_, Get(1).My) = GetSource<MyInt?, MyInt>();
+		}
+
+		public void Property_NoConversion_DiscardLast()
+		{
+			(Get(0).NMy, _) = GetSource<MyInt?, MyInt>();
+		}
 	}
 }
diff --git a/ICSharpCode.Decompiler/IL/Instructions/DeconstructInstruction.cs b/ICSharpCode.Decompiler/IL/Instructions/DeconstructInstruction.cs
index ad1b511a6..2a0cbd165 100644
--- a/ICSharpCode.Decompiler/IL/Instructions/DeconstructInstruction.cs
+++ b/ICSharpCode.Decompiler/IL/Instructions/DeconstructInstruction.cs
@@ -273,8 +273,9 @@ namespace ICSharpCode.Decompiler.IL
 				Debug.Assert(!inst.HasDesignator);
 				foreach (var subPattern in inst.SubPatterns.Cast<MatchInstruction>()) {
 					if (subPattern.IsVar) {
-						Debug.Assert(subPattern.Variable.IsSingleDefinition && subPattern.Variable.LoadCount == 1);
-						Debug.Assert(subPattern.Variable.LoadInstructions[0].IsDescendantOf(this));
+						Debug.Assert(subPattern.Variable.IsSingleDefinition && subPattern.Variable.LoadCount <= 1);
+						if (subPattern.Variable.LoadCount == 1)
+							Debug.Assert(subPattern.Variable.LoadInstructions[0].IsDescendantOf(this));
 						patternVariables.Add(subPattern.Variable);
 					} else {
 						ValidatePattern(subPattern);
diff --git a/ICSharpCode.Decompiler/IL/Transforms/DeconstructionTransform.cs b/ICSharpCode.Decompiler/IL/Transforms/DeconstructionTransform.cs
index 23f6ae238..ce6c736d4 100644
--- a/ICSharpCode.Decompiler/IL/Transforms/DeconstructionTransform.cs
+++ b/ICSharpCode.Decompiler/IL/Transforms/DeconstructionTransform.cs
@@ -19,6 +19,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Resources;
 
 using ICSharpCode.Decompiler.CSharp.Resolver;
 using ICSharpCode.Decompiler.TypeSystem;
@@ -188,7 +189,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 					return false;
 				// TODO v.LoadCount may be 2 if the deconstruction is assigned to a tuple variable
 				// or 0? because of discards
-				if (!(v.StoreCount == 0 && v.AddressCount == 1 && v.LoadCount == 1))
+				if (!(v.StoreCount == 0 && v.AddressCount == 1 && v.LoadCount <= 1))
 					return false;
 				deconstructionResultsLookup.Add(v, i - 1);
 				deconstructionResults.Add(v);
@@ -311,7 +312,11 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 
 		bool MatchAssignment(ILInstruction inst, out IType targetType, out ILInstruction valueInst, out Action<DeconstructInstruction> addAssignment)
 		{
+			targetType = null;
+			valueInst = null;
 			addAssignment = null;
+			if (inst == null)
+				return false;
 			if (inst.MatchStLoc(out var v, out var value)
 				&& value is Block block && block.MatchInlineAssignBlock(out var call, out valueInst)) {
 				if (!DeconstructInstruction.IsAssignment(call, out targetType, out _))