Print 0m instead of default(decimal) for zero decimal values

decimal has no IL literal: legacy csc compiles 0m to a Decimal.Zero
field load, which already decompiled to the literal, but Roslyn
compiles it to a zero-initialization, which decompiled to
default(decimal). The two forms are bit-identical for decimal, so the
literal is no less faithful to the IL and matches what a human writes;
it also removes the per-compiler split in the CompoundAssignmentTest
fixture.

Assisted-by: Claude:claude-fable-5:Claude Code

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/Issue2260SwitchString.cs

@@ -16,7 +16,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.ILPretty
 					break;
 				case "stock_qty":
 					{
-						decimal result2 = default(decimal);
+						decimal result2 = 0m;
 						if (!decimal.TryParse(s, out result2))
 						{
 							new object();
@@ -29,12 +29,12 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.ILPretty
 					}
 				case "new_price":
 					{
-						decimal num = default(decimal);
+						decimal num = 0m;
 						break;
 					}
 				case "new_price4":
 					{
-						decimal result4 = default(decimal);
+						decimal result4 = 0m;
 						if (decimal.TryParse(s, out result4) && !(result4 < 0m))
 						{
 						}
@@ -42,7 +42,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.ILPretty
 					}
 				case "new_price1":
 					{
-						decimal result3 = default(decimal);
+						decimal result3 = 0m;
 						if (!decimal.TryParse(s, out result3))
 						{
 							new object();
@@ -57,7 +57,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.ILPretty
 					}
 				case "new_price2":
 					{
-						decimal result = default(decimal);
+						decimal result = 0m;
 						if (!decimal.TryParse(s, out result))
 						{
 							new object();

ICSharpCode.Decompiler.Tests/TestCases/Pretty/CompoundAssignmentTest.cs

@@ -4406,11 +4406,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		}
 		public static void DecimalAddTest(decimal p, CustomClass c, CustomStruct2 s)
 		{
-#if LEGACY_CSC
 			decimal num = 0m;
-#else
-			decimal num = default(decimal);
-#endif
 			p += 5m;
 			num += 5m;
 			Use(ref num);
@@ -4437,11 +4433,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 		public static void DecimalSubtractTest(decimal p, CustomClass c, CustomStruct2 s)
 		{
-#if LEGACY_CSC
 			decimal num = 0m;
-#else
-			decimal num = default(decimal);
-#endif
 			p -= 5m;
 			num -= 5m;
 			Use(ref num);
@@ -4468,11 +4460,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 		public static void DecimalMultiplyTest(decimal p, CustomClass c, CustomStruct2 s)
 		{
-#if LEGACY_CSC
 			decimal num = 0m;
-#else
-			decimal num = default(decimal);
-#endif
 			p *= 5m;
 			num *= 5m;
 			Use(ref num);
@@ -4499,11 +4487,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 		public static void DecimalDivideTest(decimal p, CustomClass c, CustomStruct2 s)
 		{
-#if LEGACY_CSC
 			decimal num = 0m;
-#else
-			decimal num = default(decimal);
-#endif
 			p /= 5m;
 			num /= 5m;
 			Use(ref num);
@@ -4530,11 +4514,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 		public static void DecimalModulusTest(decimal p, CustomClass c, CustomStruct2 s)
 		{
-#if LEGACY_CSC
 			decimal num = 0m;
-#else
-			decimal num = default(decimal);
-#endif
 			p %= 5m;
 			num %= 5m;
 			Use(ref num);
@@ -4561,11 +4541,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 		public static void DecimalPostIncTest(decimal p, CustomClass c, CustomStruct2 s)
 		{
-#if LEGACY_CSC
 			decimal num = 0m;
-#else
-			decimal num = default(decimal);
-#endif
 			X(p++);
 			X(num++);
 			Use(ref num);
@@ -4592,11 +4568,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 		public static void DecimalPreIncTest(decimal p, CustomClass c, CustomStruct2 s)
 		{
-#if LEGACY_CSC
 			decimal num = 0m;
-#else
-			decimal num = default(decimal);
-#endif
 			X(++p);
 			X(++num);
 			Use(ref num);
@@ -4622,11 +4594,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		}
 		public static void DecimalPostDecTest(decimal p, CustomClass c, CustomStruct2 s)
 		{
-#if LEGACY_CSC
 			decimal num = 0m;
-#else
-			decimal num = default(decimal);
-#endif
 			X(p--);
 			X(num--);
 			Use(ref num);
@@ -4653,11 +4621,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 		public static void DecimalPreDecTest(decimal p, CustomClass c, CustomStruct2 s)
 		{
-#if LEGACY_CSC
 			decimal num = 0m;
-#else
-			decimal num = default(decimal);
-#endif
 			X(--p);
 			X(--num);
 			Use(ref num);

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -697,6 +697,13 @@ namespace ICSharpCode.Decompiler.CSharp
 				return new CastExpression(ConvertType(type), expr.WithRR(crr))
 					.WithRR(new ConversionResolveResult(type, crr, Conversion.NullLiteralConversion));
 			}
+			else if (type.IsKnownType(KnownTypeCode.Decimal))
+			{
+				expr = new PrimitiveExpression(0m);
+				constantType = type;
+				constantValue = 0m;
+				return expr.WithRR(new ConstantResolveResult(constantType, constantValue));
+			}
 			else
 			{
 				expr = new DefaultValueExpression(ConvertType(type));