Add support for string constant patterns.

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

@@ -335,6 +335,17 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			}
 		}
 
+		public void RecursivePattern_StringConstant(object obj)
+		{
+			if (obj is X { B: "Hello" } x)
+			{
+				Console.WriteLine("Test " + x);
+			}
+			else
+			{
+				Console.WriteLine("not Test");
+			}
+		}
 #endif
 		private bool F()
 		{

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -4641,6 +4641,8 @@ namespace ICSharpCode.Decompiler.CSharp
 						default:
 							throw new InvalidOperationException("Unexpected comparison kind: " + comp.Kind);
 					}
+				case Call call when MatchInstruction.IsCallToString_op_Equality(call):
+					return Translate(call.Arguments[1]);
 				default:
 					throw new NotImplementedException();
 			}

ICSharpCode.Decompiler/IL/Instructions/MatchInstruction.cs

@@ -135,12 +135,22 @@ namespace ICSharpCode.Decompiler.IL
 						testedOperand = comp.Left;
 						return IsConstant(comp.Right);
 					}
+				case Call call when IsCallToString_op_Equality(call):
+					testedOperand = call.Arguments[0];
+					return call.Arguments[1].OpCode == OpCode.LdStr;
 				default:
 					testedOperand = null;
 					return false;
 			}
 		}
 
+		internal static bool IsCallToString_op_Equality(Call call)
+		{
+			return call.Method.IsOperator && call.Method.Name == "op_Equality"
+				&& call.Method.DeclaringType.IsKnownType(KnownTypeCode.String)
+				&& call.Arguments.Count == 2;
+		}
+
 		private static bool IsConstant(ILInstruction inst)
 		{
 			return inst.OpCode switch {
@@ -150,7 +160,6 @@ namespace ICSharpCode.Decompiler.IL
 				OpCode.LdcI4 => true,
 				OpCode.LdcI8 => true,
 				OpCode.LdNull => true,
-				OpCode.LdStr => true,
 				_ => false
 			};
 		}