Add `OutVarResolveResult` to overload resolution (not yet used by CallBuilder)

ICSharpCode.Decompiler.Tests/TestCases/Correctness/OverloadResolution.cs

@@ -34,6 +34,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 			TestIndexer();
 			Issue1281();
 			Issue1747();
+			CallAmbiguousOutParam();
 		}
 
 		#region ConstructorTest
@@ -246,6 +247,29 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 			obj[5] = null;
 		}
 		#endregion
+
+		#region Out Parameter
+		static void AmbiguousOutParam(out string a)
+		{
+			a = null;
+			Console.WriteLine("AmbiguousOutParam(out string)");
+		}
+
+		static void AmbiguousOutParam(out int b)
+		{
+			b = 1;
+			Console.WriteLine("AmbiguousOutParam(out int)");
+		}
+
+		static void CallAmbiguousOutParam()
+		{
+			Console.WriteLine("CallAmbiguousOutParam:");
+			string a;
+			int b;
+			AmbiguousOutParam(out a);
+			AmbiguousOutParam(out b);
+		}
+		#endregion
 	}
 
 	class IndexerTests

ICSharpCode.Decompiler/CSharp/Resolver/OverloadResolution.cs

@@ -649,12 +649,18 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 					continue;
 				}
 
-				ByReferenceResolveResult brrr = arguments[i] as ByReferenceResolveResult;
-				if (brrr != null)
+				if (arguments[i] is ByReferenceResolveResult brrr)
 				{
 					if (brrr.ReferenceKind != candidate.Parameters[parameterIndex].ReferenceKind)
 						candidate.AddError(OverloadResolutionErrors.ParameterPassingModeMismatch);
 				}
+				else if (arguments[i] is OutVarResolveResult)
+				{
+					if (candidate.Parameters[parameterIndex].ReferenceKind != ReferenceKind.Out)
+						candidate.AddError(OverloadResolutionErrors.ParameterPassingModeMismatch);
+					// 'out var decl' arguments are compatible with any out parameter
+					continue;
+				}
 				else
 				{
 					if (candidate.Parameters[parameterIndex].ReferenceKind != ReferenceKind.None)

ICSharpCode.Decompiler/ICSharpCode.Decompiler.csproj

@@ -86,6 +86,7 @@
     <Compile Include="Documentation\XmlDocumentationElement.cs" />
     <Compile Include="IL\ControlFlow\AwaitInFinallyTransform.cs" />
     <Compile Include="IL\Transforms\IntroduceNativeIntTypeOnLocals.cs" />
+    <Compile Include="Semantics\OutVarResolveResult.cs" />
     <Compile Include="Solution\ProjectId.cs" />
     <Compile Include="Solution\ProjectItem.cs" />
     <Compile Include="Solution\SolutionCreator.cs" />

ICSharpCode.Decompiler/Semantics/OutVarResolveResult.cs

@@ -0,0 +1,31 @@
+// Copyright (c) 2020 Daniel Grunwald
+// 
+// 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.
+
+using ICSharpCode.Decompiler.TypeSystem;
+
+namespace ICSharpCode.Decompiler.Semantics
+{
+	/// <summary>
+	/// Represents the implicitly-typed "out var".
+	/// Special-cased in overload resolution to be compatible with any out-parameter.
+	/// </summary>
+	class OutVarResolveResult : ResolveResult
+	{
+		public OutVarResolveResult() : base(SpecialType.NoType) { }
+	}
+}