Add DetectImprovedTypeForForBitOp as a transform step.

Previously this happened within the InferType implementation.

ICSharpCode.Decompiler/IL/Instructions/BinaryNumericInstruction.cs

@@ -78,6 +78,14 @@ namespace ICSharpCode.Decompiler.IL
 
 		readonly StackType resultType;
 
+		/// <summary>
+		/// Result type of this instruction. If not null, InferType() will return this type.
+		/// Can be used to store an improved C# type for the result of this instruction,
+		/// e.g. `bit.or.i4(bool1, bool2)` can use CSharpResultType = bool after the
+		/// ExpressionTransform has determined that the result is always 0 or 1.
+		/// </summary>
+		public IType? CSharpResultType { get; set; }
+
 		public BinaryNumericInstruction(BinaryNumericOperator op, ILInstruction left, ILInstruction right, bool checkForOverflow, Sign sign)
 			: this(op, left, right, left.ResultType, right.ResultType, checkForOverflow, sign)
 		{
@@ -123,7 +131,7 @@ namespace ICSharpCode.Decompiler.IL
 			return StackType.Unknown;
 		}
 
-		public StackType UnderlyingResultType { get => resultType; }
+		public StackType UnderlyingResultType => resultType;
 
 		public sealed override StackType ResultType {
 			get => IsLifted ? StackType.O : resultType;
@@ -131,6 +139,8 @@ namespace ICSharpCode.Decompiler.IL
 
 		public override IType InferType(ICompilation compilation)
 		{
+			if (CSharpResultType != null)
+				return CSharpResultType;
 			IType type = compilation.FindType(UnderlyingResultType);
 			if (IsLifted)
 				return NullableType.Create(compilation, type);

ICSharpCode.Decompiler/IL/Transforms/ExpressionTransforms.cs

@@ -863,8 +863,12 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 					}
 					break;
 				case BinaryNumericOperator.BitAnd:
-					if (inst.Left.InferType(context.TypeSystem).IsKnownType(KnownTypeCode.Boolean)
-						&& inst.Right.InferType(context.TypeSystem).IsKnownType(KnownTypeCode.Boolean))
+				{
+					IType leftType = inst.Left.InferType(context.TypeSystem);
+					IType rightType = inst.Right.InferType(context.TypeSystem);
+					DetectImprovedTypeForForBitOp(inst, leftType, rightType);
+					if (leftType.IsKnownType(KnownTypeCode.Boolean)
+						&& rightType.IsKnownType(KnownTypeCode.Boolean))
 					{
 						if (new NullableLiftingTransform(context).Run(inst))
 						{
@@ -872,6 +876,15 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 						}
 					}
 					break;
+				}
+				case BinaryNumericOperator.BitOr:
+				case BinaryNumericOperator.BitXor:
+				{
+					IType leftType = inst.Left.InferType(context.TypeSystem);
+					IType rightType = inst.Right.InferType(context.TypeSystem);
+					DetectImprovedTypeForForBitOp(inst, leftType, rightType);
+					break;
+				}
 			}
 
 			bool MatchExpectedShiftSize(ILInstruction rhs)
@@ -894,6 +907,20 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			}
 		}
 
+		// Given bit.or(a, b), promotes the "known to be bool/short" information from the operands to bit.or
+		void DetectImprovedTypeForForBitOp(BinaryNumericInstruction inst, IType leftType, IType rightType)
+		{
+			if (leftType.Equals(rightType) && (leftType.IsCSharpPrimitiveIntegerType() || leftType.IsCSharpNativeIntegerType() || leftType.IsKnownType(KnownTypeCode.Boolean)))
+			{
+				if (!leftType.Equals(inst.CSharpResultType))
+				{
+					context.Step("DetectImprovedTypeForForBitOp", inst);
+					inst.CSharpResultType = leftType;
+					context.EndStep(inst);
+				}
+			}
+		}
+
 		protected internal override void VisitTryCatchHandler(TryCatchHandler inst)
 		{
 			base.VisitTryCatchHandler(inst);