Report nullable warnings for receiver of pattern indexers (#37216)

Fixes #36241

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs

@@ -6036,7 +6036,12 @@ public override BoundNode VisitIndexerAccess(BoundIndexerAccess node)
 
         public override BoundNode VisitIndexOrRangePatternIndexerAccess(BoundIndexOrRangePatternIndexerAccess node)
         {
-            var receiverType = VisitRvalueWithState(node.Receiver);
+            BoundExpression receiver = node.Receiver;
+            var receiverType = VisitRvalueWithState(receiver);
+            // https://github.com/dotnet/roslyn/issues/30598: Mark receiver as not null
+            // after indices have been visited, and only if the receiver has not changed.
+            _ = CheckPossibleNullReceiver(receiver);
+
             VisitRvalue(node.Argument);
             var patternSymbol = node.PatternSymbol;
             if (!receiverType.HasNullType)

src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs

@@ -101387,6 +101387,33 @@ static void F3(int? z, int? w)
             comp.VerifyDiagnostics();
         }
 
+        [Fact]
+        public void PatternIndexer()
+        {
+            var src = @"
+#nullable enable
+class C
+{
+  static void M1(string? s)
+  {
+    _ = s[^1];
+  }
+  
+  static void M2(string? s)
+  {
+    _ = s[1..10];
+  }
+}";
+            var comp = CreateCompilationWithIndexAndRange(src, options: WithNonNullTypesTrue());
+            comp.VerifyDiagnostics(
+                // (7,9): warning CS8602: Dereference of a possibly null reference.
+                //     _ = s[^1];
+                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "s").WithLocation(7, 9),
+                // (12,9): warning CS8602: Dereference of a possibly null reference.
+                //     _ = s[1..10];
+                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "s").WithLocation(12, 9));
+        }
+
         [WorkItem(31770, "https://github.com/dotnet/roslyn/issues/31770")]
         [Fact]
         public void UserDefinedConversion_NestedNullability_01()