Fix "Remove redundant comp(... != 0)" to avoid changing the value

When an IfInstruction was created with resultType=bool, because only
values 0 or 1 are possible, then we need to preserve this property in
transforms -- otherwise the added correctness test would fail.

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

@@ -52,6 +52,12 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 			Console.WriteLine(IsNull(new OverloadedOperators()));
 			Console.WriteLine(NullIs(new OverloadedOperators()));
 			Console.WriteLine(NullIsNot(new OverloadedOperators()));
+
+			Console.WriteLine("IntBranchInConditionSlot:");
+			IntBranchInConditionSlot(1, 0x100, false);
+			IntBranchInConditionSlot(0, 0x100, true);
+			NegatedIntBranchInConditionSlot(1, 0x100, false);
+			NegatedIntBranchInConditionSlot(0, 0x100, true);
 			return 0;
 		}
 
@@ -92,6 +98,26 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 			Console.WriteLine("uint: {0} == Id(-1) = {1}", i, i == Id(-1));
 		}
 
+		// The conditional has an int-valued true branch and a bool false branch, so it is
+		// compiled to an I4 value that the surrounding 'if' tests for non-zero. Converting
+		// that value to bool must keep the non-zero test: truncating it to 8 bits first
+		// loses every bit from 8 up, and flags = 0x100 then reads as false.
+		static void IntBranchInConditionSlot(int x, int flags, bool other)
+		{
+			if ((x > 0) ? (flags != 0) : other)
+				Console.WriteLine("true");
+			else
+				Console.WriteLine("false");
+		}
+
+		static void NegatedIntBranchInConditionSlot(int x, int flags, bool other)
+		{
+			if (!((x > 0) ? (flags != 0) : other))
+				Console.WriteLine("negated: false");
+			else
+				Console.WriteLine("negated: true");
+		}
+
 		static void Issue2398(long value)
 		{
 			if ((int)value != 0)

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceUsingDeclarations.cs

@@ -396,8 +396,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				if (astBuilder.NameLookupMode == NameLookupMode.Type)
 				{
 					AstType outermostType = simpleType;
-					while (outermostType.Parent is AstType)
-						outermostType = (AstType)outermostType.Parent;
+					while (outermostType.Parent is AstType parent)
+						outermostType = parent;
 					if (outermostType.Parent is TypeReferenceExpression)
 					{
 						// ILSpy uses TypeReferenceExpression in expression context even when the C# parser

ICSharpCode.Decompiler/IL/Instructions/IfInstruction.cs

@@ -123,6 +123,12 @@ namespace ICSharpCode.Decompiler.IL
 				}
 			}
 			output.Write(OpCode);
+			if (resultType != null)
+			{
+				output.Write(" [");
+				output.Write(resultType.ReflectionName);
+				output.Write(']');
+			}
 			output.Write(" (");
 			condition.WriteTo(output, options);
 			output.Write(") ");

ICSharpCode.Decompiler/IL/Transforms/ExpressionTransforms.cs

@@ -96,10 +96,12 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				return;
 			}
 			else if (inst.Kind == ComparisonKind.Inequality && inst.LiftingKind == ComparisonLiftingKind.None
-				&& inst.Right.MatchLdcI4(0) && (IfInstruction.IsInConditionSlot(inst) || inst.Left is Comp))
+				&& inst.Right.MatchLdcI4(0)
+				&& (inst.Left.InferType(context.TypeSystem).IsKnownType(KnownTypeCode.Boolean)
+				   || inst.Left.MatchLdcI4(0) || inst.Left.MatchLdcI4(1)))
 			{
-				// if (comp(x != 0)) ==> if (x)
-				// comp(comp(...) != 0) => comp(...)
+				// When `x` is known to be 0 or 1:
+				// `comp(x != 0) => x`
 				context.Step("Remove redundant comp(... != 0)", inst);
 				inst.Left.AddILRange(inst);
 				var left = inst.Left;