Add test

src/EditorFeatures/CSharpTest/UseIndexOrRangeOperator/UseIndexOperatorTests.cs

@@ -264,16 +264,26 @@ void Goo(S s)
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsUseIndexOperator)]
         public async Task TestUserDefinedTypeWithNoIndexIndexer()
         {
-            await TestMissingAsync(
+            await TestInRegularAndScript1Async(
 @"
 namespace System { public struct Index { } }
-struct S { public int Length { get; } public int this[int i] { get; } }
+struct S { public int Count { get; } public int this[int i] { get; } }
 class C
 {
     void Goo(S s)
     {
         var v = s[[||]s.Count - 2];
     }
+}",
+@"
+namespace System { public struct Index { } }
+struct S { public int Count { get; } public int this[int i] { get; } }
+class C
+{
+    void Goo(S s)
+    {
+        var v = s[^2];
+    }
 }", parameters: s_testParameters);
         }
 
@@ -500,6 +510,32 @@ void Goo(List<int> s)
 }", parseOptions: s_parseOptions);
         }
 
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsUseIndexOperator)]
+        public async Task TestSimple_NoIndexIndexer_SupportsIntIndexer_Set()
+        {
+            await TestAsync(
+@"
+using System.Collections.Generic;
+namespace System { public struct Index { } }
+class C
+{
+    void Goo(List<int> s)
+    {
+        s[[||]s.Count - 1] = 1;
+    }
+}",
+@"
+using System.Collections.Generic;
+namespace System { public struct Index { } }
+class C
+{
+    void Goo(List<int> s)
+    {
+        s[^1] = 1;
+    }
+}", parseOptions: s_parseOptions);
+        }
+
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsUseIndexOperator)]
         public async Task NotOnConstructedIndexer()
         {

src/EditorFeatures/CSharpTest/UseIndexOrRangeOperator/UseRangeOperatorTests.cs

@@ -144,6 +144,21 @@ void Goo(string s)
 }", parseOptions: s_parseOptions);
         }
 
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsUseRangeOperator)]
+        public async Task TestNotWithoutSubtraction()
+        {
+            await TestMissingInRegularAndScriptAsync(
+@"
+namespace System { public struct Range { } }
+class C
+{
+    void Goo(string s)
+    {
+        var v = s.Substring([||]1, 2);
+    }
+}", new TestParameters(parseOptions: s_parseOptions));
+        }
+
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsUseRangeOperator)]
         public async Task TestNonStringType()
         {