Escape identifier "_" to avoid ambiguities with discard patterns.

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

@@ -23,6 +23,10 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 {
 	public class TypeAnalysisTests
 	{
+		private class @_
+		{
+		}
+
 		private byte[] byteArray;
 
 		public byte SubtractFrom256(byte b)
@@ -123,6 +127,8 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		{
 			Console.WriteLine(o is Random);
 			Console.WriteLine(!(o is Random));
+			// If we didn't escape the '_' identifier here, this would look like a discard pattern
+			Console.WriteLine(o is @_);
 		}
 
 		public byte[] CreateArrayWithInt(int length)

ICSharpCode.Decompiler.Tests/TestCases/Pretty/TypeAnalysisTests.il

@@ -32,6 +32,21 @@
 .class public auto ansi beforefieldinit ICSharpCode.Decompiler.Tests.TestCases.Pretty.TypeAnalysisTests
        extends [mscorlib]System.Object
 {
+  .class auto ansi nested private beforefieldinit _
+         extends [mscorlib]System.Object
+  {
+    .method public hidebysig specialname rtspecialname 
+            instance void  .ctor() cil managed
+    {
+      // Code size       7 (0x7)
+      .maxstack  8
+      IL_0000:  ldarg.0
+      IL_0001:  call       instance void [mscorlib]System.Object::.ctor()
+      IL_0006:  ret
+    } // end of method _::.ctor
+
+  } // end of class _
+
   .field private uint8[] byteArray
   .method public hidebysig instance uint8 
           SubtractFrom256(uint8 b) cil managed
@@ -392,7 +407,7 @@
   .method public hidebysig instance void 
           OperatorIs(object o) cil managed
   {
-    // Code size       35 (0x23)
+    // Code size       50 (0x32)
     .maxstack  8
     IL_0000:  nop
     IL_0001:  ldarg.1
@@ -409,7 +424,13 @@
     IL_001a:  ceq
     IL_001c:  call       void [mscorlib]System.Console::WriteLine(bool)
     IL_0021:  nop
-    IL_0022:  ret
+    IL_0022:  ldarg.1
+    IL_0023:  isinst     ICSharpCode.Decompiler.Tests.TestCases.Pretty.TypeAnalysisTests/_
+    IL_0028:  ldnull
+    IL_0029:  cgt.un
+    IL_002b:  call       void [mscorlib]System.Console::WriteLine(bool)
+    IL_0030:  nop
+    IL_0031:  ret
   } // end of method TypeAnalysisTests::OperatorIs
 
   .method public hidebysig instance uint8[] 

ICSharpCode.Decompiler.Tests/TestCases/Pretty/TypeAnalysisTests.opt.il

@@ -32,6 +32,21 @@
 .class public auto ansi beforefieldinit ICSharpCode.Decompiler.Tests.TestCases.Pretty.TypeAnalysisTests
        extends [mscorlib]System.Object
 {
+  .class auto ansi nested private beforefieldinit _
+         extends [mscorlib]System.Object
+  {
+    .method public hidebysig specialname rtspecialname 
+            instance void  .ctor() cil managed
+    {
+      // Code size       7 (0x7)
+      .maxstack  8
+      IL_0000:  ldarg.0
+      IL_0001:  call       instance void [mscorlib]System.Object::.ctor()
+      IL_0006:  ret
+    } // end of method _::.ctor
+
+  } // end of class _
+
   .field private uint8[] byteArray
   .method public hidebysig instance uint8 
           SubtractFrom256(uint8 b) cil managed
@@ -278,7 +293,7 @@
   .method public hidebysig instance void 
           OperatorIs(object o) cil managed
   {
-    // Code size       32 (0x20)
+    // Code size       46 (0x2e)
     .maxstack  8
     IL_0000:  ldarg.1
     IL_0001:  isinst     [mscorlib]System.Random
@@ -292,7 +307,12 @@
     IL_0017:  ldc.i4.0
     IL_0018:  ceq
     IL_001a:  call       void [mscorlib]System.Console::WriteLine(bool)
-    IL_001f:  ret
+    IL_001f:  ldarg.1
+    IL_0020:  isinst     ICSharpCode.Decompiler.Tests.TestCases.Pretty.TypeAnalysisTests/_
+    IL_0025:  ldnull
+    IL_0026:  cgt.un
+    IL_0028:  call       void [mscorlib]System.Console::WriteLine(bool)
+    IL_002d:  ret
   } // end of method TypeAnalysisTests::OperatorIs
 
   .method public hidebysig instance uint8[] 

ICSharpCode.Decompiler.Tests/TestCases/Pretty/TypeAnalysisTests.opt.roslyn.il

@@ -36,6 +36,21 @@
 .class public auto ansi beforefieldinit ICSharpCode.Decompiler.Tests.TestCases.Pretty.TypeAnalysisTests
        extends [mscorlib]System.Object
 {
+  .class auto ansi nested private beforefieldinit _
+         extends [mscorlib]System.Object
+  {
+    .method public hidebysig specialname rtspecialname 
+            instance void  .ctor() cil managed
+    {
+      // Code size       7 (0x7)
+      .maxstack  8
+      IL_0000:  ldarg.0
+      IL_0001:  call       instance void [mscorlib]System.Object::.ctor()
+      IL_0006:  ret
+    } // end of method _::.ctor
+
+  } // end of class _
+
   .field private uint8[] byteArray
   .method public hidebysig instance uint8 
           SubtractFrom256(uint8 b) cil managed
@@ -282,7 +297,7 @@
   .method public hidebysig instance void 
           OperatorIs(object o) cil managed
   {
-    // Code size       32 (0x20)
+    // Code size       46 (0x2e)
     .maxstack  8
     IL_0000:  ldarg.1
     IL_0001:  isinst     [mscorlib]System.Random
@@ -296,7 +311,12 @@
     IL_0017:  ldc.i4.0
     IL_0018:  ceq
     IL_001a:  call       void [mscorlib]System.Console::WriteLine(bool)
-    IL_001f:  ret
+    IL_001f:  ldarg.1
+    IL_0020:  isinst     ICSharpCode.Decompiler.Tests.TestCases.Pretty.TypeAnalysisTests/_
+    IL_0025:  ldnull
+    IL_0026:  cgt.un
+    IL_0028:  call       void [mscorlib]System.Console::WriteLine(bool)
+    IL_002d:  ret
   } // end of method TypeAnalysisTests::OperatorIs
 
   .method public hidebysig instance uint8[] 

ICSharpCode.Decompiler.Tests/TestCases/Pretty/TypeAnalysisTests.roslyn.il

@@ -36,6 +36,22 @@
 .class public auto ansi beforefieldinit ICSharpCode.Decompiler.Tests.TestCases.Pretty.TypeAnalysisTests
        extends [mscorlib]System.Object
 {
+  .class auto ansi nested private beforefieldinit _
+         extends [mscorlib]System.Object
+  {
+    .method public hidebysig specialname rtspecialname 
+            instance void  .ctor() cil managed
+    {
+      // Code size       8 (0x8)
+      .maxstack  8
+      IL_0000:  ldarg.0
+      IL_0001:  call       instance void [mscorlib]System.Object::.ctor()
+      IL_0006:  nop
+      IL_0007:  ret
+    } // end of method _::.ctor
+
+  } // end of class _
+
   .field private uint8[] byteArray
   .method public hidebysig instance uint8 
           SubtractFrom256(uint8 b) cil managed
@@ -393,7 +409,7 @@
   .method public hidebysig instance void 
           OperatorIs(object o) cil managed
   {
-    // Code size       35 (0x23)
+    // Code size       50 (0x32)
     .maxstack  8
     IL_0000:  nop
     IL_0001:  ldarg.1
@@ -410,7 +426,13 @@
     IL_001a:  ceq
     IL_001c:  call       void [mscorlib]System.Console::WriteLine(bool)
     IL_0021:  nop
-    IL_0022:  ret
+    IL_0022:  ldarg.1
+    IL_0023:  isinst     ICSharpCode.Decompiler.Tests.TestCases.Pretty.TypeAnalysisTests/_
+    IL_0028:  ldnull
+    IL_0029:  cgt.un
+    IL_002b:  call       void [mscorlib]System.Console::WriteLine(bool)
+    IL_0030:  nop
+    IL_0031:  ret
   } // end of method TypeAnalysisTests::OperatorIs
 
   .method public hidebysig instance uint8[] 

ICSharpCode.Decompiler/CSharp/OutputVisitor/CSharpOutputVisitor.cs

@@ -395,9 +395,13 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		public static bool IsKeyword(string identifier, AstNode context)
 		{
 			// only 2-10 char lower-case identifiers can be keywords
-			if (identifier.Length > maxKeywordLength || identifier.Length < 2 || identifier[0] < 'a') {
+			if (identifier.Length > maxKeywordLength || identifier.Length == 0) {
 				return false;
 			}
+			if (identifier[0] < 'a') {
+				// we escape _ as identifier to avoid ambiguities with discard patterns
+				return identifier == "_";
+			}
 			if (unconditionalKeywords.Contains(identifier)) {
 				return true;
 			}