implement special cases for enum type inference; closes #248

ICSharpCode.Decompiler/ILAst/TypeAnalysis.cs

@@ -1004,6 +1004,19 @@ namespace ICSharpCode.Decompiler.ILAst
 				InferTypeForExpression(right, typeSystem.IntPtr);
 				return leftPreferred;
 			}
+			if (IsEnum(leftPreferred)) {
+				if (expectedType != null && IsEnum(expectedType)) {
+					// E-U=E
+					left.InferredType = left.ExpectedType = leftPreferred;
+					InferTypeForExpression(right, GetEnumUnderlyingType(leftPreferred));
+					return leftPreferred;
+				} else {
+					// E-E=U
+					left.InferredType = left.ExpectedType = leftPreferred;
+					InferTypeForExpression(right, leftPreferred);
+					return GetEnumUnderlyingType(leftPreferred);
+				}
+			}
 			return InferBinaryArguments(left, right, expectedType, leftPreferred: leftPreferred);
 		}
 

ICSharpCode.Decompiler/Tests/TypeAnalysisTests.cs

@@ -140,4 +140,14 @@ public class TypeAnalysisTests
 	{
 		return new byte[length];
 	}
+	
+	public StringComparison EnumDiffNumber(StringComparison data)
+	{
+		return data - 1;
+	}
+	
+	public int EnumDiff(StringComparison a, StringComparison b)
+	{
+		return Math.Abs(a - b);
+	}
 }