From 753f1d2481a26ad12c2b621632f9bcce4fbf4c04 Mon Sep 17 00:00:00 2001
From: Siegfried Pammer <siegfriedpammer@gmail.com>
Date: Mon, 20 Jul 2026 21:40:27 +0200
Subject: [PATCH] Fix #1924: don't undo the compiler's &&-to-& optimization

ExpressionBuilder printed bitwise & / | on booleans as && / || whenever
the right-hand side was pure, but Roslyn lowers && / || to & / | only
when the right operand is a bare local or parameter read
(LocalRewriter.MakeBinaryOperator, unchanged since 2014). Shapes like
(c == 'a') | (c == 'b') can therefore only originate from a bitwise
source operator, yet were shown as short-circuiting. Per the discussion
in #1545, show the operator the IL actually uses instead of guessing the
source form: the reversal is dropped entirely, so Roslyn-compiled
"a && b" now decompiles to "a & b", which recompiles to the same IL.

Assisted-by: Claude:claude-fable-5:Claude Code
---
 .../TestCases/Pretty/ShortCircuit.cs           | 18 +++++++++++++-----
 .../TestCases/Pretty/Switch.cs                 |  8 ++++----
 .../CSharp/ExpressionBuilder.cs                | 16 ----------------
 3 files changed, 17 insertions(+), 25 deletions(-)

diff --git a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/ShortCircuit.cs b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/ShortCircuit.cs
index 9516ad3cf..d2193890b 100644
--- a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/ShortCircuit.cs
+++ b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/ShortCircuit.cs
@@ -326,13 +326,21 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		}
 #endif
 
-		public void PreferLogicalToBitwise(bool a, bool b, int i, float f)
+		public void BitwiseBooleanOperators(bool a, bool b, int i, float f)
 		{
-			B(a && b);
+			B(a & b);
+			B(a & (i == 1));
+			B((i == 1) & a);
+			B((i > i - 3) & a);
+			B((f < 0.1f) & a);
+			B(a | b);
 			B(a && i == 1);
-			B(i == 1 && a);
-			B(i > i - 3 && a);
-			B(f < 0.1f && a);
+			B(a || i == 1);
+		}
+
+		public bool BitwiseOrWithComparisons(char c)
+		{
+			return (c == 'a') | (c == 'b');
 		}
 	}
 }
diff --git a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/Switch.cs b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/Switch.cs
index 2fac42565..a5e5a3bbb 100644
--- a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/Switch.cs
+++ b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/Switch.cs
@@ -1282,7 +1282,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 #if ROSLYN || OPT
 		public static void SingleIf1(int i, bool a)
 		{
-			if (i == 1 || (i == 2 && a))
+			if (i == 1 || ((i == 2) & a))
 			{
 				Console.WriteLine(1);
 			}
@@ -1292,7 +1292,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 		public static void SingleIf2(int i, bool a, bool b)
 		{
-			if (i == 1 || (i == 2 && a) || (i == 3 && b))
+			if (i == 1 || ((i == 2) & a) || ((i == 3) & b))
 			{
 				Console.WriteLine(1);
 			}
@@ -1301,7 +1301,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 		public static void SingleIf3(int i, bool a, bool b)
 		{
-			if (a || i == 1 || (i == 2 && b))
+			if (a || i == 1 || ((i == 2) & b))
 			{
 				Console.WriteLine(1);
 			}
@@ -1310,7 +1310,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 		public static void SingleIf4(int i, bool a)
 		{
-			if (i == 1 || i == 2 || (i != 3 && a) || i != 4)
+			if (i == 1 || i == 2 || ((i != 3) & a) || i != 4)
 			{
 				Console.WriteLine(1);
 			}
diff --git a/ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs b/ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs
index f2504ac9c..ddd5da25b 100644
--- a/ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs
+++ b/ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs
@@ -1650,22 +1650,6 @@ namespace ICSharpCode.Decompiler.CSharp
 						right.ResolveResult));
 				}
 			}
-			if (op.IsBitwise()
-				&& left.Type.IsKnownType(KnownTypeCode.Boolean)
-				&& right.Type.IsKnownType(KnownTypeCode.Boolean)
-				&& SemanticHelper.IsPure(inst.Right.Flags))
-			{
-				// Undo the C# compiler's optimization of "a && b" to "a & b".
-				if (op == BinaryOperatorType.BitwiseAnd)
-				{
-					op = BinaryOperatorType.ConditionalAnd;
-				}
-				else if (op == BinaryOperatorType.BitwiseOr)
-				{
-					op = BinaryOperatorType.ConditionalOr;
-				}
-			}
-
 			if (op.IsBitwise() && (left.Type.Kind == TypeKind.Enum || right.Type.Kind == TypeKind.Enum))
 			{
 				left = AdjustConstantExpressionToType(left, right.Type);