Use original defs

src/EditorFeatures/CSharpTest/UseIndexOrRangeOperator/UseIndexOperatorTests.cs

@@ -499,5 +499,21 @@ void Goo(List<int> s)
     }
 }", parseOptions: s_parseOptions);
         }
+
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsUseIndexOperator)]
+        public async Task NotOnConstructedIndexer()
+        {
+            await TestMissingInRegularAndScriptAsync(
+@"
+using System.Collections.Generic;
+namespace System { public struct Index { } }
+class C
+{
+    void Goo(Dictionary<int, string> s)
+    {
+        var v = s[[||]s.Count - 1];
+    }
+}", new TestParameters(parseOptions: s_parseOptions));
+        }
     }
 }

src/Features/CSharp/Portable/UseIndexOrRangeOperator/Helpers.cs

@@ -76,7 +76,11 @@ public static bool IsIntIndexingMethod(IMethodSymbol method)
                (method.MethodKind == MethodKind.PropertyGet || method.MethodKind == MethodKind.Ordinary) &&
                IsPublicInstance(method) &&
                method.Parameters.Length == 1 &&
-               method.Parameters[0].Type.SpecialType == SpecialType.System_Int32;
+               // From: https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/ranges.md#decisions-made-during-implementation
+               //
+               // When looking for the pattern members, we look for original definitions, not
+               // constructed members
+               method.OriginalDefinition.Parameters[0].Type.SpecialType == SpecialType.System_Int32;
 
         /// <summary>
         /// Look for methods like "SomeType MyType.Slice(int start, int length)".  Note that the
@@ -87,8 +91,12 @@ public static bool IsSliceLikeMethod(IMethodSymbol method)
             => method != null &&
                IsPublicInstance(method) &&
                method.Parameters.Length == 2 &&
-               IsSliceFirstParameter(method.Parameters[0]) &&
-               IsSliceSecondParameter(method.Parameters[1]);
+               // From: https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/ranges.md#decisions-made-during-implementation
+               //
+               // When looking for the pattern members, we look for original definitions, not
+               // constructed members
+               IsSliceFirstParameter(method.OriginalDefinition.Parameters[0]) &&
+               IsSliceSecondParameter(method.OriginalDefinition.Parameters[1]);
 
         private static bool IsSliceFirstParameter(IParameterSymbol parameter)
             => parameter.Type.SpecialType == SpecialType.System_Int32 &&