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

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)

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
 		};