From 4a2afbb7236f8c061a953562650d222edaaa09db Mon Sep 17 00:00:00 2001
From: Siegfried Pammer <siegfriedpammer@gmail.com>
Date: Mon, 10 Aug 2026 18:56:00 +0200
Subject: [PATCH] Walk indexer accesses when looking for a null-conditional
 source

A query source can be reached through an indexer as well as through a member
access or a call: `holder?[0].Where(...).Select(...)` puts an IndexerExpression
between the LINQ call and the `?.`. The receiver walk stopped there, so query
syntax was still introduced over a source the conditional access had lifted to
a nullable value type, and the output failed to compile with CS1936 - the same
way as the case that was reported, one node kind further along.

IndexerExpression.Target is nullable where MemberReferenceExpression's and
InvocationExpression's are not, so only that arm needs to match on the target.

Assisted-by: Claude:claude-opus-5[1m]:Claude Code
---
 .../TestCases/Pretty/QueryExpressions.cs                 | 9 +++++++++
 .../CSharp/Transforms/IntroduceQueryExpressions.cs       | 3 ++-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/QueryExpressions.cs b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/QueryExpressions.cs
index 160ef4a65..0870735b1 100644
--- a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/QueryExpressions.cs
+++ b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/QueryExpressions.cs
@@ -48,6 +48,10 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		{
 			public Maybe<int> Value;
 
+#if CS60
+			public Maybe<int> this[int index] => default(Maybe<int>);
+#endif
+
 			public Func<Maybe<int>> Factory()
 			{
 				return () => default(Maybe<int>);
@@ -237,6 +241,11 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		{
 			return holder?.Factory()().Where((int value) => value > 0).Select((int value) => value.ToString());
 		}
+
+		public Maybe<string>? NullConditionalIndexerQuery(MaybeHolder holder)
+		{
+			return holder?[0].Where((int value) => value > 0).Select((int value) => value.ToString());
+		}
 #endif
 
 		public static IEnumerable<char> Issue1310a(bool test)
diff --git a/ICSharpCode.Decompiler/CSharp/Transforms/IntroduceQueryExpressions.cs b/ICSharpCode.Decompiler/CSharp/Transforms/IntroduceQueryExpressions.cs
index d33e41884..4745061e3 100644
--- a/ICSharpCode.Decompiler/CSharp/Transforms/IntroduceQueryExpressions.cs
+++ b/ICSharpCode.Decompiler/CSharp/Transforms/IntroduceQueryExpressions.cs
@@ -380,7 +380,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		bool IsNullConditional(Expression target) => target switch {
 			UnaryOperatorExpression { Operator: UnaryOperatorType.NullConditional } => true,
 			MemberReferenceExpression member => IsNullConditional(member.Target),
-			InvocationExpression { Target: { } invocationTarget } => IsNullConditional(invocationTarget),
+			InvocationExpression invocation => IsNullConditional(invocation.Target),
+			IndexerExpression { Target: { } indexerTarget } => IsNullConditional(indexerTarget),
 			_ => false
 		};