Do not re-emit initializer stores of propagated display-class fields

A field that is propagated to the variable it was initialized from is
replaced by that variable, so re-emitting its initializer assigns the
variable to itself. Where the field was initialized from 'this' the
result does not even compile ('this = this'). The store is dropped
instead, which is what VisitStObj already does for initializer stores
that are not part of an object-initializer block; the insertion position
has to be tracked separately from the loop index, because skipping a
store would otherwise push the following ones past the end of the block.

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

ICSharpCode.Decompiler.Tests/TestCases/Ugly/AggressiveScalarReplacementOfAggregates.Expected.cs

@@ -184,5 +184,12 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Ugly
 			Interlocked.Exchange(ref thisField, new Program());
 			Console.WriteLine("{0} {1}", this, thisField);
 		}
+
+		public void Test11()
+		{
+			int field1 = 1;
+			string field2 = "Hello World!";
+			Console.WriteLine("{0} {1} {2}", this, field1, field2);
+		}
 	}
 }

ICSharpCode.Decompiler.Tests/TestCases/Ugly/AggressiveScalarReplacementOfAggregates.cs

@@ -202,5 +202,15 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Ugly
 			Interlocked.Exchange(ref displayClass.thisField, new Program());
 			Console.WriteLine("{0} {1}", this, displayClass.thisField);
 		}
+
+		public void Test11()
+		{
+			DisplayClass displayClass = new DisplayClass {
+				thisField = this,
+				field1 = 1,
+				field2 = "Hello World!"
+			};
+			Console.WriteLine("{0} {1} {2}", displayClass.thisField, displayClass.field1, displayClass.field2);
+		}
 	}
 }

ICSharpCode.Decompiler/IL/Transforms/TransformDisplayClassUsage.cs

@@ -820,12 +820,23 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 					{
 						context.Step($"Remove initializer of {inst.Variable.Name}", inst);
 						ILInstruction firstInlinedStore = null;
+						// Stores are appended after the initializer, in source order; a store that
+						// is dropped must not leave a gap, so the position is tracked separately
+						// from the loop index.
+						int insertionIndex = inst.ChildIndex;
 						for (int i = 1; i < initBlock.Instructions.Count; i++)
 						{
 							var stobj = (StObj)initBlock.Instructions[i];
 							var variable = displayClass.VariablesToDeclare[(IField)((LdFlda)stobj.Target).Field.MemberDefinition];
+							// A propagated field is replaced by the variable it was initialized from,
+							// so keeping its initializer would assign that variable to itself - and
+							// where the variable is 'this', the result is not even valid C#. Stores
+							// outside an object-initializer block are dropped in VisitStObj under the
+							// same condition.
+							if (variable.CanPropagate && variable.Initializers.Contains(stobj))
+								continue;
 							var inlinedStore = new StLoc(variable.GetOrDeclare(), stobj.Value).WithILRange(stobj);
-							parentBlock.Instructions.Insert(inst.ChildIndex + i, inlinedStore);
+							parentBlock.Instructions.Insert(++insertionIndex, inlinedStore);
 							firstInlinedStore ??= inlinedStore;
 						}
 						context.EndStep(firstInlinedStore);