Workaround dotnet/roslyn#43659 in decompiled code.

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

@@ -79,6 +79,14 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			set;
 		}
 		public event EventHandler? Event;
+
+		public static int? NullConditionalOperator(T02_EverythingIsNullableInHere? x)
+		{
+			// This code throws if `x != null && x.field1 == null`.
+			// But we can't decompile it to the warning-free "x?.field1!.Length",
+			// because of https://github.com/dotnet/roslyn/issues/43659
+			return x?.field1.Length;
+		}
 	}
 
 	public class T03_EverythingIsNotNullableInHere

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -2027,7 +2027,7 @@ namespace ICSharpCode.Decompiler.CSharp
 							.WithRR(new ResolveResult(NullableType.GetUnderlyingType(translatedTarget.Type)))
 							.WithoutILInstruction();
 					}
-					translatedTarget = EnsureTargetNotNullable(translatedTarget);
+					translatedTarget = EnsureTargetNotNullable(translatedTarget, target);
 					return translatedTarget;
 				}
 			} else {
@@ -2055,12 +2055,18 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 		}
 
-		private TranslatedExpression EnsureTargetNotNullable(TranslatedExpression expr)
+		private TranslatedExpression EnsureTargetNotNullable(TranslatedExpression expr, ILInstruction inst)
 		{
+			// inst is the instruction that got translated into expr.
 			if (expr.Type.Nullability == Nullability.Nullable) {
 				if (expr.Expression is UnaryOperatorExpression uoe && uoe.Operator == UnaryOperatorType.NullConditional) {
 					return expr;
 				}
+				if (inst.HasFlag(InstructionFlags.MayUnwrapNull)) {
+					// We can't use ! in the chain of operators after a NullConditional, due to
+					// https://github.com/dotnet/roslyn/issues/43659
+					return expr;
+				}
 				return new UnaryOperatorExpression(UnaryOperatorType.SuppressNullableWarning, expr)
 					.WithRR(new ResolveResult(expr.Type.ChangeNullability(Nullability.Oblivious)))
 					.WithoutILInstruction();
@@ -2153,7 +2159,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			if (arrayExpr.Type.Kind != TypeKind.Array) {
 				arrayExpr = arrayExpr.ConvertTo(arrayType, this);
 			}
-			arrayExpr = EnsureTargetNotNullable(arrayExpr);
+			arrayExpr = EnsureTargetNotNullable(arrayExpr, inst.Array);
 			string memberName;
 			KnownTypeCode code;
 			if (inst.ResultType == StackType.I4) {