Fix nullability analysis of property indexers

src/libraries/System.Private.CoreLib/src/System/Reflection/NullabilityInfoContext.cs

@@ -69,10 +69,30 @@ public NullabilityInfo Create(ParameterInfo parameterInfo)
         {
             ArgumentNullException.ThrowIfNull(parameterInfo);
 
+            bool annotationsDisabled = false;
+            if (parameterInfo.Member is PropertyInfo propertyInfo)
+            {
+                // For property indexers, switch to the respective getter/setter parameter.
+                if (propertyInfo.GetGetMethod(true) is { } getter && !IsPrivateOrInternalMethodAndAnnotationDisabled(getter))
+                {
+                    parameterInfo = getter.GetParametersAsSpan()[parameterInfo.Position];
+                }
+                else if (propertyInfo.GetSetMethod(true) is { } setter && !IsPrivateOrInternalMethodAndAnnotationDisabled(setter))
+                {
+                    parameterInfo = setter.GetParametersAsSpan()[parameterInfo.Position];
+                }
+                else
+                {
+                    annotationsDisabled = true;
+                }
+            }
+            else if (parameterInfo.Member is MethodBase method)
+            {
+                annotationsDisabled = IsPrivateOrInternalMethodAndAnnotationDisabled(method);
+            }
+
             IList<CustomAttributeData> attributes = parameterInfo.GetCustomAttributesData();
-            NullableAttributeStateParser parser = parameterInfo.Member is MethodBase method && IsPrivateOrInternalMethodAndAnnotationDisabled(method)
-                ? NullableAttributeStateParser.Unknown
-                : CreateParser(attributes);
+            NullableAttributeStateParser parser = annotationsDisabled ? NullableAttributeStateParser.Unknown : CreateParser(attributes);
             NullabilityInfo nullability = GetNullabilityInfo(parameterInfo.Member, parameterInfo.ParameterType, parser);
 
             if (nullability.ReadState != NullabilityState.Unknown)

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Reflection/NullabilityInfoContextTests.cs

@@ -1471,6 +1471,24 @@ public void TestMethodsWithGenericParameters(Delegate @delegate, NullabilityStat
             Assert.Equal(expectedRead, info.ReadState);
             Assert.Equal(expectedWrite, info.WriteState);
         }
+
+        public static IEnumerable<object[]> TestClassesWithIndexers() => new object[][]
+        {
+            [typeof(ClassWithOnlyIndexedGetProperty), "Item", NullabilityState.NotNull, NullabilityState.NotNull],
+            [typeof(ClassWithIndexedGetPropertyAndTupleProperty), "Item", NullabilityState.NotNull, NullabilityState.NotNull],
+            [typeof(ClassWithOnlyIndexedSetProperty), "Item", NullabilityState.NotNull, NullabilityState.NotNull],
+            [typeof(ClassWithIndexedSetPropertyAndTupleProperty), "Item", NullabilityState.NotNull, NullabilityState.NotNull],
+        };
+
+        [Theory]
+        [MemberData(nameof(TestClassesWithIndexers))]
+        public void TestPropertyIndexer(Type type, string propertyName, NullabilityState expectedRead, NullabilityState expectedWrite)
+        {
+            PropertyInfo property = type.GetProperty(propertyName)!;
+            NullabilityInfo info = nullabilityContext.Create(property.GetIndexParameters()[0]);
+            Assert.Equal(expectedRead, info.ReadState);
+            Assert.Equal(expectedWrite, info.WriteState);
+        }
     }
 
 #pragma warning disable CS0649, CS0067, CS0414
@@ -1806,4 +1824,22 @@ public class ClassWithGenericMethods_Allow
         public static void GenericMethod<T>([AllowNull] T value) => throw new Exception();
         public static void GenericNotNullMethod<T>([AllowNull] T value) where T : notnull => throw new Exception();
     }
+    public class ClassWithOnlyIndexedGetProperty
+    {
+        public string this[string name] { get => throw new Exception(); }
+    }
+    public class ClassWithIndexedGetPropertyAndTupleProperty
+    {
+        public string this[string name] { get => throw new Exception(); }
+        public (int A, int B) ValueTupleProp { get; }
+    }
+    public class ClassWithOnlyIndexedSetProperty
+    {
+        public string this[string name] { set => throw new Exception(); }
+    }
+    public class ClassWithIndexedSetPropertyAndTupleProperty
+    {
+        public string this[string name] { set => throw new Exception(); }
+        public (int A, int B) ValueTupleProp { get; }
+    }
 }