Fix LambdaResolveResult.IsValid() being too restrictive.

This could cause our overload resolution to consider an overload as not-applicable when it actually is applicable. This could cause us to miss some cases where we need to insert casts.

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

@@ -17,6 +17,7 @@
 // DEALINGS IN THE SOFTWARE.
 
 using System;
+using System.Linq;
 
 namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 {
@@ -27,6 +28,8 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 			InterestingConstants();
 			TruncatedComp();
 			StringConcat();
+			LinqNullableMin();
+			LinqNullableMin(1, 2, 3);
 		}
 
 		static void Print<T>(T val)
@@ -101,5 +104,11 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 			Print(string.Concat(1, 2));
 			Print(string.Concat(1, 2, "str"));
 		}
+
+		static void LinqNullableMin(params int[] arr)
+		{
+			Print(string.Format("LinqNullableMin {0}:", arr.Length));
+			Print(arr.Min(v => (int?)v));
+		}
 	}
 }

ICSharpCode.Decompiler/CSharp/Resolver/LambdaResolveResult.cs

@@ -162,7 +162,8 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 					}
 				}
 			}
-			if (conversions.IdentityConversion(returnType, this.ReturnType)) {
+			if (conversions.IdentityConversion(this.ReturnType, returnType)
+				|| conversions.ImplicitConversion(this.InferredReturnType, returnType).IsValid) {
 				return LambdaConversion.Instance;
 			} else {
 				return Conversion.None;