Fix #2140: ILSpy 6.2p1 no longer respects "use discards" setting

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

@@ -14,6 +14,16 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			value = 0;
 		}
 
+		public void GetOutOverloaded(out int value)
+		{
+			value = 0;
+		}
+
+		public void GetOutOverloaded(out string value)
+		{
+			value = "Hello World";
+		}
+
 		public void MakeValue(Func<object, string, int> func)
 		{
 
@@ -28,16 +38,21 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		{
 		}
 
-		// out _ is currently not supported: the test cases below are not useful
-		//public void ParameterHiddenByLocal(@_ _)
-		//{
-		//	GetOut(out int value);
-		//}
+		public void ParameterHiddenByLocal(@_ _)
+		{
+			GetOut(out var _);
+		}
 
-		//public void DiscardedOutVsLambdaParameter()
-		//{
-		//	GetOut(out int value);
-		//	MakeValue((@_ _) => 5);
-		//}
+		public void DiscardedOutVsLambdaParameter()
+		{
+			GetOut(out var _);
+			MakeValue((@_ _) => 5);
+		}
+
+		public void ExplicitlyTypedDiscard()
+		{
+			GetOutOverloaded(out string _);
+			GetOutOverloaded(out int _);
+		}
 	}
 }

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

@@ -82,7 +82,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		private static void RefCallSiteTests()
 		{
 #if CS70
-			CallWithOut(out dynamic d);
+			CallWithOut(out var d);
 			CallWithIn(in d);
 #else
 			dynamic d;

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

@@ -1040,7 +1040,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		{
 			await Task.Delay(100);
 #if CS70
-			if (string.IsNullOrEmpty(str) && int.TryParse(str, out int id))
+			if (string.IsNullOrEmpty(str) && int.TryParse(str, out var id))
 			{
 #else
 			int id;

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

@@ -25,7 +25,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 	{
 		public static void OutVarInShortCircuit(Dictionary<int, string> d)
 		{
-			if (d.Count > 2 && d.TryGetValue(42, out string value))
+			if (d.Count > 2 && d.TryGetValue(42, out var value))
 			{
 				Console.WriteLine(value);
 			}
@@ -35,7 +35,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		{
 			// Note: needs reasoning about "definitely assigned if true"
 			// to ensure that the value is initialized when the delegate is declared.
-			if (d.Count > 2 && d.TryGetValue(42, out string value))
+			if (d.Count > 2 && d.TryGetValue(42, out var value))
 			{
 				return delegate {
 					Console.WriteLine(value);

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

@@ -32,7 +32,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 			private static int? GetIntOrNull(string v)
 			{
-				if (int.TryParse(v, out int result))
+				if (int.TryParse(v, out var result))
 				{
 					return result;
 				}

ICSharpCode.Decompiler/CSharp/Annotations.cs

@@ -345,4 +345,12 @@ namespace ICSharpCode.Decompiler.CSharp
 			this.EqualsLambda = equals;
 		}
 	}
+
+	/// <summary>
+	/// Annotates an out DirectionExpression if the out variable can be declared implicitly typed.
+	/// </summary>
+	public class UseImplicitlyTypedOutAnnotation
+	{
+		public static readonly UseImplicitlyTypedOutAnnotation Instance = new UseImplicitlyTypedOutAnnotation();
+	}
 }

ICSharpCode.Decompiler/CSharp/CallBuilder.cs

@@ -52,6 +52,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			public IReadOnlyList<int> ArgumentToParameterMap;
 
 			public bool AddNamesToPrimitiveValues;
+			public bool UseImplicitlyTypedOut;
 			public bool IsExpandedForm;
 			public int Length => Arguments.Length;
 
@@ -62,9 +63,33 @@ namespace ICSharpCode.Decompiler.CSharp
 				return FirstOptionalArgumentIndex;
 			}
 
-			public IEnumerable<ResolveResult> GetArgumentResolveResults(int skipCount = 0)
+			public IList<ResolveResult> GetArgumentResolveResults(int skipCount = 0)
 			{
-				return Arguments.Skip(skipCount).Take(GetActualArgumentCount()).Select(a => a.ResolveResult);
+				var expectedParameters = ExpectedParameters;
+				var useImplicitlyTypedOut = UseImplicitlyTypedOut;
+
+				return Arguments
+					.SelectWithIndex(GetResolveResult)
+					.Skip(skipCount)
+					.Take(GetActualArgumentCount())
+					.ToArray();
+
+				ResolveResult GetResolveResult(int index, TranslatedExpression expression)
+				{
+					var param = expectedParameters[index];
+					if (useImplicitlyTypedOut && param.IsOut)
+						return OutVarResolveResult.Instance;
+					return expression.ResolveResult;
+				}
+			}
+
+			public IList<ResolveResult> GetArgumentResolveResultsDirect(int skipCount = 0)
+			{
+				return Arguments
+					.Skip(skipCount)
+					.Take(GetActualArgumentCount())
+					.Select(a => a.ResolveResult)
+					.ToArray();
 			}
 
 			public IEnumerable<Expression> GetArgumentExpressions(int skipCount = 0)
@@ -87,9 +112,10 @@ namespace ICSharpCode.Decompiler.CSharp
 					}
 				}
 				int argumentCount = GetActualArgumentCount();
+				var useImplicitlyTypedOut = UseImplicitlyTypedOut;
 				if (ArgumentNames == null)
 				{
-					return Arguments.Skip(skipCount).Take(argumentCount).Select(arg => arg.Expression);
+					return Arguments.Skip(skipCount).Take(argumentCount).Select(arg => AddAnnotations(arg.Expression));
 				}
 				else
 				{
@@ -97,11 +123,23 @@ namespace ICSharpCode.Decompiler.CSharp
 					return Arguments.Take(argumentCount).Zip(ArgumentNames.Take(argumentCount),
 						(arg, name) => {
 							if (name == null)
-								return arg.Expression;
+								return AddAnnotations(arg.Expression);
 							else
-								return new NamedArgumentExpression(name, arg);
+								return new NamedArgumentExpression(name, AddAnnotations(arg.Expression));
 						});
 				}
+
+				Expression AddAnnotations(Expression expression)
+				{
+					if (!useImplicitlyTypedOut)
+						return expression;
+					if (expression.GetResolveResult() is ByReferenceResolveResult brrr
+						&& brrr.IsOut)
+					{
+						expression.AddAnnotation(UseImplicitlyTypedOutAnnotation.Instance);
+					}
+					return expression;
+				}
 			}
 
 			public bool CanInferAnonymousTypePropertyNamesFromArguments()
@@ -255,13 +293,14 @@ namespace ICSharpCode.Decompiler.CSharp
 			{
 				return new InvocationExpression(target, argumentList.GetArgumentExpressions())
 					.WithRR(new CSharpInvocationResolveResult(target.ResolveResult, method,
-						argumentList.GetArgumentResolveResults().ToList(), isExpandedForm: argumentList.IsExpandedForm));
+						argumentList.GetArgumentResolveResults(), isExpandedForm: argumentList.IsExpandedForm));
 			}
 
 			if (method is VarArgInstanceMethod)
 			{
 				argumentList.FirstOptionalArgumentIndex = -1;
 				argumentList.AddNamesToPrimitiveValues = false;
+				argumentList.UseImplicitlyTypedOut = false;
 				int regularParameterCount = ((VarArgInstanceMethod)method).RegularParameterCount;
 				var argListArg = new UndocumentedExpression();
 				argListArg.UndocumentedExpressionType = UndocumentedExpressionType.ArgList;
@@ -293,7 +332,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			{
 				return new InvocationExpression(target, argumentList.GetArgumentExpressions())
 					.WithRR(new CSharpInvocationResolveResult(target.ResolveResult, method,
-						argumentList.GetArgumentResolveResults().ToList(), isExpandedForm: argumentList.IsExpandedForm, isDelegateInvocation: true));
+						argumentList.GetArgumentResolveResults(), isExpandedForm: argumentList.IsExpandedForm, isDelegateInvocation: true));
 			}
 
 			if (settings.StringInterpolation && IsInterpolatedStringCreation(method, argumentList) &&
@@ -340,7 +379,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					}
 					var formattableStringType = expressionBuilder.compilation.FindType(KnownTypeCode.FormattableString);
 					var isrr = new InterpolatedStringResolveResult(expressionBuilder.compilation.FindType(KnownTypeCode.String),
-						format, argumentList.GetArgumentResolveResults().Skip(1).ToArray());
+						format, argumentList.GetArgumentResolveResults(1).ToArray());
 					var expr = new InterpolatedStringExpression();
 					expr.Content.AddRange(content);
 					if (method.Name == "Format")
@@ -363,7 +402,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				argumentList.CheckNoNamedOrOptionalArguments();
 				return HandleDelegateEqualityComparison(method, argumentList.Arguments)
 					.WithRR(new CSharpInvocationResolveResult(target.ResolveResult, method,
-						argumentList.GetArgumentResolveResults().ToList(), isExpandedForm: argumentList.IsExpandedForm));
+						argumentList.GetArgumentResolveResults(), isExpandedForm: argumentList.IsExpandedForm));
 			}
 
 			if (method.IsOperator && method.Name == "op_Implicit" && argumentList.Length == 1)
@@ -420,7 +459,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				typeArgumentList.AddRange(method.TypeArguments.Select(expressionBuilder.ConvertType));
 			return new InvocationExpression(targetExpr, argumentList.GetArgumentExpressions())
 				.WithRR(new CSharpInvocationResolveResult(target.ResolveResult, foundMethod,
-					argumentList.GetArgumentResolveResults().ToList(), isExpandedForm: argumentList.IsExpandedForm));
+					argumentList.GetArgumentResolveResultsDirect(), isExpandedForm: argumentList.IsExpandedForm));
 		}
 
 		/// <summary>
@@ -457,6 +496,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				firstParamIndex: 0, args, null);
 			argumentList.ArgumentNames = null;
 			argumentList.AddNamesToPrimitiveValues = false;
+			argumentList.UseImplicitlyTypedOut = false;
 			var transform = GetRequiredTransformationsForCall(expectedTargetDetails, method, ref unused,
 				ref argumentList, CallTransformation.None, out _);
 			Debug.Assert(transform == CallTransformation.None || transform == CallTransformation.NoOptionalArgumentAllowed);
@@ -755,6 +795,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			list.IsExpandedForm = isExpandedForm;
 			list.IsPrimitiveValue = isPrimitiveValue;
 			list.FirstOptionalArgumentIndex = firstOptionalArgumentIndex;
+			list.UseImplicitlyTypedOut = true;
 			list.AddNamesToPrimitiveValues = expressionBuilder.settings.NamedArguments && expressionBuilder.settings.NonTrailingNamedArguments;
 			return list;
 		}
@@ -787,7 +828,8 @@ namespace ICSharpCode.Decompiler.CSharp
 					}
 				}
 				if (IsUnambiguousCall(expectedTargetDetails, method, targetResolveResult, Empty<IType>.Array,
-					expandedArguments, argumentNames: null, firstOptionalArgumentIndex: -1, out _,
+					expandedArguments.SelectArray(a => a.ResolveResult), argumentNames: null,
+					firstOptionalArgumentIndex: -1, out _,
 					out var bestCandidateIsExpandedForm) == OverloadResolutionErrors.None && bestCandidateIsExpandedForm)
 				{
 					expectedParameters = expandedParameters;
@@ -917,7 +959,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			bool argumentsCasted = false;
 			OverloadResolutionErrors errors;
 			while ((errors = IsUnambiguousCall(expectedTargetDetails, method, targetResolveResult, typeArguments,
-				argumentList.Arguments, argumentList.ArgumentNames, argumentList.FirstOptionalArgumentIndex, out foundMethod,
+				argumentList.GetArgumentResolveResults().ToArray(), argumentList.ArgumentNames, argumentList.FirstOptionalArgumentIndex, out foundMethod,
 				out var bestCandidateIsExpandedForm)) != OverloadResolutionErrors.None || bestCandidateIsExpandedForm != argumentList.IsExpandedForm)
 			{
 				switch (errors)
@@ -958,6 +1000,7 @@ namespace ICSharpCode.Decompiler.CSharp
 								appliedRequireTypeArgumentsShortcut = false;
 							}
 							argumentsCasted = true;
+							argumentList.UseImplicitlyTypedOut = false;
 							CastArguments(argumentList.Arguments, argumentList.ExpectedParameters);
 						}
 						else if ((allowedTransforms & CallTransformation.RequireTarget) != 0 && !requireTarget)
@@ -1116,27 +1159,32 @@ namespace ICSharpCode.Decompiler.CSharp
 		}
 
 		OverloadResolutionErrors IsUnambiguousCall(ExpectedTargetDetails expectedTargetDetails, IMethod method,
-			ResolveResult target, IType[] typeArguments, IList<TranslatedExpression> arguments,
+			ResolveResult target, IType[] typeArguments, ResolveResult[] arguments,
 			string[] argumentNames, int firstOptionalArgumentIndex,
 			out IParameterizedMember foundMember, out bool bestCandidateIsExpandedForm)
 		{
 			foundMember = null;
 			bestCandidateIsExpandedForm = false;
-			var lookup = new MemberLookup(resolver.CurrentTypeDefinition, resolver.CurrentTypeDefinition.ParentModule);
+			var currentTypeDefinition = resolver.CurrentTypeDefinition;
+			var lookup = new MemberLookup(currentTypeDefinition, currentTypeDefinition.ParentModule);
 
 			Log.WriteLine("IsUnambiguousCall: Performing overload resolution for " + method);
-			Log.WriteCollection("  Arguments: ", arguments.Select(a => a.ResolveResult));
+			Log.WriteCollection("  Arguments: ", arguments);
+
+			argumentNames = firstOptionalArgumentIndex < 0 || argumentNames == null
+				? argumentNames
+				: argumentNames.Take(firstOptionalArgumentIndex).ToArray();
 
 			var or = new OverloadResolution(resolver.Compilation,
-				firstOptionalArgumentIndex < 0 ? arguments.SelectArray(a => a.ResolveResult) : arguments.Take(firstOptionalArgumentIndex).Select(a => a.ResolveResult).ToArray(),
-				argumentNames: firstOptionalArgumentIndex < 0 || argumentNames == null ? argumentNames : argumentNames.Take(firstOptionalArgumentIndex).ToArray(),
-				typeArguments: typeArguments,
+				arguments, argumentNames, typeArguments,
 				conversions: expressionBuilder.resolver.conversions);
 			if (expectedTargetDetails.CallOpCode == OpCode.NewObj)
 			{
 				foreach (IMethod ctor in method.DeclaringType.GetConstructors())
 				{
-					if (lookup.IsAccessible(ctor, allowProtectedAccess: resolver.CurrentTypeDefinition == method.DeclaringTypeDefinition))
+					bool allowProtectedAccess =
+						resolver.CurrentTypeDefinition == method.DeclaringTypeDefinition;
+					if (lookup.IsAccessible(ctor, allowProtectedAccess))
 					{
 						or.AddCandidate(ctor);
 					}
@@ -1145,12 +1193,12 @@ namespace ICSharpCode.Decompiler.CSharp
 			else if (method.IsOperator)
 			{
 				IEnumerable<IParameterizedMember> operatorCandidates;
-				if (arguments.Count == 1)
+				if (arguments.Length == 1)
 				{
 					IType argType = NullableType.GetUnderlyingType(arguments[0].Type);
 					operatorCandidates = resolver.GetUserDefinedOperatorCandidates(argType, method.Name);
 				}
-				else if (arguments.Count == 2)
+				else if (arguments.Length == 2)
 				{
 					IType lhsType = NullableType.GetUnderlyingType(arguments[0].Type);
 					IType rhsType = NullableType.GetUnderlyingType(arguments[1].Type);
@@ -1170,7 +1218,8 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 			else if (target == null)
 			{
-				var result = resolver.ResolveSimpleName(method.Name, typeArguments, isInvocationTarget: true) as MethodGroupResolveResult;
+				var result = resolver.ResolveSimpleName(method.Name, typeArguments, isInvocationTarget: true)
+					as MethodGroupResolveResult;
 				if (result == null)
 					return OverloadResolutionErrors.AmbiguousMatch;
 				or.AddMethodLists(result.MethodsGroupedByDeclaringType.ToArray());
@@ -1386,13 +1435,14 @@ namespace ICSharpCode.Decompiler.CSharp
 					}
 				}
 				return atce.WithRR(new CSharpInvocationResolveResult(
-					target, method, argumentList.GetArgumentResolveResults().ToList(),
+					target, method, argumentList.GetArgumentResolveResults(),
 					isExpandedForm: argumentList.IsExpandedForm, argumentToParameterMap: argumentList.ArgumentToParameterMap
 				));
 			}
 			else
 			{
-				while (IsUnambiguousCall(expectedTargetDetails, method, null, Empty<IType>.Array, argumentList.Arguments,
+				while (IsUnambiguousCall(expectedTargetDetails, method, null, Empty<IType>.Array,
+					argumentList.GetArgumentResolveResults().ToArray(),
 					argumentList.ArgumentNames, argumentList.FirstOptionalArgumentIndex, out _,
 					out var bestCandidateIsExpandedForm) != OverloadResolutionErrors.None || bestCandidateIsExpandedForm != argumentList.IsExpandedForm)
 				{

ICSharpCode.Decompiler/CSharp/Transforms/DeclareVariables.cs

@@ -580,17 +580,36 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				}
 				else if (CanBeDeclaredAsOutVariable(v, out var dirExpr))
 				{
-					// 'T v; SomeCall(out v);' can be combined to 'SomeCall(out var v);'
+					// 'T v; SomeCall(out v);' can be combined to 'SomeCall(out T v);'
 					AstType type;
 					if (context.Settings.AnonymousTypes && v.Type.ContainsAnonymousType())
 					{
 						type = new SimpleType("var");
 					}
+					else if (dirExpr.Annotation<UseImplicitlyTypedOutAnnotation>() != null)
+					{
+						type = new SimpleType("var");
+					}
 					else
 					{
 						type = context.TypeSystemAstBuilder.ConvertType(v.Type);
 					}
-					var ovd = new OutVarDeclarationExpression(type, v.Name);
+					string name;
+					// Variable is not used and discards are allowed, we can simplify this to 'out T _'.
+					// TODO: if no variable named _ is declared and var is used instead of T, use out _.
+					// Note: ExpressionBuilder.HidesVariableWithName produces inaccurate results, because it
+					// does not take lambdas and local functions into account, that are defined in the same
+					// scope as v.
+					if (context.Settings.Discards && v.ILVariable.LoadCount == 0
+						&& v.ILVariable.StoreCount == 0 && v.ILVariable.AddressCount == 1)
+					{
+						name = "_";
+					}
+					else
+					{
+						name = v.Name;
+					}
+					var ovd = new OutVarDeclarationExpression(type, name);
 					ovd.Variable.AddAnnotation(new ILVariableResolveResult(ilVariable));
 					ovd.CopyAnnotationsFrom(dirExpr);
 					replacements.Add((dirExpr, ovd));

ICSharpCode.Decompiler/Semantics/OutVarResolveResult.cs

@@ -26,6 +26,8 @@ namespace ICSharpCode.Decompiler.Semantics
 	/// </summary>
 	class OutVarResolveResult : ResolveResult
 	{
+		public static readonly OutVarResolveResult Instance = new OutVarResolveResult();
+
 		public OutVarResolveResult() : base(SpecialType.NoType) { }
 	}
 }