support implicit indexing pattern

src/EditorFeatures/CSharpTest/UseIndexOrRangeOperator/UseIndexOperatorTests.cs

@@ -471,6 +471,32 @@ void Goo(string[] s)
     {
         var v1 = s[^2][^1];
     }
+}", parseOptions: s_parseOptions);
+        }
+
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsUseIndexOperator)]
+        public async Task TestSimple_NoIndexIndexer_SupportsIntIndexer()
+        {
+            await TestAsync(
+@"
+using System.Collections.Generic;
+namespace System { public struct Index { } }
+class C
+{
+    void Goo(List<int> s)
+    {
+        var v = s[[||]s.Count - 1];
+    }
+}",
+@"
+using System.Collections.Generic;
+namespace System { public struct Index { } }
+class C
+{
+    void Goo(List<int> s)
+    {
+        var v = s[^1];
+    }
 }", parseOptions: s_parseOptions);
         }
     }

src/Features/CSharp/Portable/UseIndexOrRangeOperator/CSharpUseIndexOperatorDiagnosticAnalyzer.InfoCache.cs

@@ -83,15 +83,14 @@ private MemberInfo ComputeMemberInfo(IMethodSymbol method, bool requireIndexMemb
                 if (method.MethodKind == MethodKind.PropertyGet)
                 {
                     // this is the getter for an indexer.  i.e. the user is calling something
-                    // like s[...].  We need to see if there's an indexer that takes a System.Index
-                    // value.
-                    var indexer = GetIndexer(containingType, IndexType, method.ReturnType);
-                    if (indexer != null)
-                    {
-                        // Type had a matching indexer.  We can convert calls to the int-indexer to
-                        // calls to this System.Index-indexer.
-                        return new MemberInfo(lengthLikeProperty, overloadedMethodOpt: null);
-                    }
+                    // like s[...].
+                    //
+                    // These can always be converted to use a System.Index.  Either because the
+                    // type itself has a System.Index-based indexer, or because the language just
+                    // allows types to implicitly seem like they support this through:
+                    //
+                    // https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/ranges.md#implicit-index-support
+                    return new MemberInfo(lengthLikeProperty, overloadedMethodOpt: null);
                 }
                 else
                 {