Respond to PR comments

src/Compilers/CSharp/Portable/Binder/Binder_Expressions.cs

@@ -6804,9 +6804,8 @@ private BoundExpression BindArrayAccess(ExpressionSyntax node, BoundExpression e
                 }
             }
 
-            HashSet<DiagnosticInfo> useSiteDiagnostics = null;
             var resultType = rank == 1 &&
-                convertedArguments[0].Type == GetWellKnownType(WellKnownType.System_Range, ref useSiteDiagnostics)
+                convertedArguments[0].Type == Compilation.GetWellKnownType(WellKnownType.System_Range)
                 ? arrayType
                 : arrayType.ElementType.TypeSymbol;
 
@@ -6839,7 +6838,7 @@ private BoundExpression ConvertToArrayIndex(BoundExpression index, SyntaxNode no
                     TryImplicitConversionToArrayIndex(index, WellKnownType.System_Range, node, diagnostics);
             }
 
-            if (result == null)
+            if (result is null)
             {
                 // Give the error that would be given upon conversion to int32.
                 NamedTypeSymbol int32 = GetSpecialType(SpecialType.System_Int32, diagnostics, node);
@@ -6860,19 +6859,19 @@ private BoundExpression TryImplicitConversionToArrayIndex(BoundExpression expr,
             HashSet<DiagnosticInfo> useSiteDiagnostics = null;
             TypeSymbol type = GetWellKnownType(wellKnownType, ref useSiteDiagnostics);
 
-            if (!type.IsErrorType())
+            if (type.IsErrorType())
             {
-                var attemptDiagnostics = DiagnosticBag.GetInstance();
-                var result = TryImplicitConversionToArrayIndex(expr, type, node, attemptDiagnostics);
-                if (!(result is null))
-                {
-                    diagnostics.AddRange(attemptDiagnostics);
-                }
-                attemptDiagnostics.Free();
-                return result;
+                return null;
             }
 
-            return null;
+            var attemptDiagnostics = DiagnosticBag.GetInstance();
+            var result = TryImplicitConversionToArrayIndex(expr, type, node, attemptDiagnostics);
+            if (!(result is null))
+            {
+                diagnostics.AddRange(attemptDiagnostics);
+            }
+            attemptDiagnostics.Free();
+            return result;
         }
 
         private BoundExpression TryImplicitConversionToArrayIndex(BoundExpression expr, SpecialType specialType, SyntaxNode node, DiagnosticBag diagnostics)
@@ -7157,19 +7156,18 @@ private BoundExpression BindIndexedPropertyAccess(SyntaxNode syntax, BoundExpres
 
                 if (!analyzedArguments.HasErrors)
                 {
-                    // PROTOTYPE: REMOVE BEFORE SHIPPING DEV16
+                    // https://github.com/dotnet/roslyn/issues/30620
                     // Pretend like there are indexers that support range and index
                     if (receiverOpt?.Type.SpecialType == SpecialType.System_String &&
                         analyzedArguments.Arguments.Count == 1)
                     {
-                        HashSet<DiagnosticInfo> diags = null;
                         var argType = analyzedArguments.Arguments[0].Type;
                         TypeSymbol resultType = null;
-                        if (argType == GetWellKnownType(WellKnownType.System_Index, ref diags))
+                        if (argType == Compilation.GetWellKnownType(WellKnownType.System_Index))
                         {
                             resultType = GetSpecialType(SpecialType.System_Char, diagnostics, syntax);
                         }
-                        else if (argType == GetWellKnownType(WellKnownType.System_Range, ref diags))
+                        else if (argType == Compilation.GetWellKnownType(WellKnownType.System_Range))
                         {
                             resultType = GetSpecialType(SpecialType.System_String, diagnostics, syntax);
                         }
@@ -7177,7 +7175,8 @@ private BoundExpression BindIndexedPropertyAccess(SyntaxNode syntax, BoundExpres
                         if (!(resultType is null))
                         {
                             var args = analyzedArguments.Arguments.ToImmutable();
-                            
+
+                            overloadResolutionResult.Free();
                             return new BoundIndexerAccess(
                                 syntax,
                                 receiverOpt,

src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter.cs

@@ -527,21 +527,22 @@ public override BoundNode VisitTypeOrInstanceInitializers(BoundTypeOrInstanceIni
 
         public override BoundNode VisitArrayAccess(BoundArrayAccess node)
         {
+            // https://github.com/dotnet/roslyn/issues/30620
             // If the array access index is of type System.Index or System.Range
-            // we need to translate it into a call into an actual indexer, instead
+            // we need to emit code as if there were a real indexer, instead
             // of a simple array element access.
 
-            if (this.EmitModule is null || 
-                node.Indices.Length != 1 ||
-                node.Indices[0].Type.SpecialType != SpecialType.None)
+            if (node.Indices.Length != 1)
             {
                 return base.VisitArrayAccess(node);
             }
 
-            // PROTOTYPE: THIS MUST BE REMOVED BEFORE DEV16
-            // Since it takes a long time to update mscorlib we fake up the right
-            // code to pretend there are two indexers that support Index and Range
-            // on arrays.
+            TypeSymbol rawIndexType = node.Indices[0].Type;
+            if (!(rawIndexType == _compilation.GetWellKnownType(WellKnownType.System_Index) ||
+                  rawIndexType == _compilation.GetWellKnownType(WellKnownType.System_Range)))
+            {
+                return base.VisitArrayAccess(node);
+            }
 
             var syntax = node.Syntax;
             var F = _factory;
@@ -603,7 +604,7 @@ public override BoundNode VisitArrayAccess(BoundArrayAccess node)
                             BinaryOperatorKind.Subtraction,
                             F.SpecialType(SpecialType.System_Int32),
                             F.ArrayLength(arrayLocal),
-                            F.Property(F.Property(indexLocal, rangeEndSymbol), indexValueSymbol)),
+                            F.Property(F.Property(indexLocal, rangeStartSymbol), indexValueSymbol)),
                         F.Property(F.Property(indexLocal, rangeStartSymbol), indexValueSymbol),
                         F.SpecialType(SpecialType.System_Int32)),
                     out BoundAssignmentOperator startAssign);

src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_IndexerAccess.cs

@@ -83,7 +83,7 @@ private BoundExpression VisitIndexerAccess(BoundIndexerAccess node, bool isLeftO
             // NOTE: This is done later by MakeArguments, for now we just lower each argument.
             ImmutableArray<BoundExpression> rewrittenArguments = VisitList(node.Arguments);
 
-            // PROTOTYPE: REMOVE BEFORE SHIPPING DEV16
+            // https://github.com/dotnet/roslyn/issues/30620
             if (rewrittenReceiver?.Type.SpecialType == SpecialType.System_String &&
                 rewrittenArguments.Length == 1 && rewrittenArguments[0].Type.SpecialType == SpecialType.None)
             {
@@ -111,13 +111,13 @@ private BoundExpression VisitIndexerAccess(BoundIndexerAccess node, bool isLeftO
                             stringAssign),
                         F.Conditional(
                             F.Property(indexLocal, indexFromEndSymbol),
-                            F.Property(stringLocal, node.Indexer,
+                            F.Indexer(stringLocal, node.Indexer,
                                 F.Binary(
                                     BinaryOperatorKind.Subtraction,
                                     F.SpecialType(SpecialType.System_Int32),
                                     F.Call(stringLocal, F.SpecialMethod(SpecialMember.System_String__Length)),
                                     indexValueExpr)),
-                            F.Property(stringLocal, node.Indexer, indexValueExpr),
+                            F.Indexer(stringLocal, node.Indexer, indexValueExpr),
                             F.SpecialType(SpecialType.System_Char)));
                 }
                 else if (argType == _compilation.GetWellKnownType(WellKnownType.System_Range))
@@ -137,7 +137,7 @@ private BoundExpression VisitIndexerAccess(BoundIndexerAccess node, bool isLeftO
                                 BinaryOperatorKind.Subtraction,
                                 F.SpecialType(SpecialType.System_Int32),
                                 F.Call(stringLocal, F.SpecialMethod(SpecialMember.System_String__Length)),
-                                F.Property(F.Property(indexLocal, rangeEndSymbol), indexValueSymbol)),
+                                F.Property(F.Property(indexLocal, rangeStartSymbol), indexValueSymbol)),
                             F.Property(F.Property(indexLocal, rangeStartSymbol), indexValueSymbol),
                             F.SpecialType(SpecialType.System_Int32)),
                         out BoundAssignmentOperator startAssign);

src/Compilers/CSharp/Portable/Lowering/SyntheticBoundNodeFactory.cs

@@ -288,7 +288,7 @@ public BoundExpression Property(BoundExpression receiverOpt, PropertySymbol prop
             return Call(receiverOpt, property.GetMethod); // TODO: should we use property.GetBaseProperty().GetMethod to ensure we generate a call to the overridden method?
         }
 
-        public BoundExpression Property(BoundExpression receiverOpt, PropertySymbol property, BoundExpression arg0)
+        public BoundExpression Indexer(BoundExpression receiverOpt, PropertySymbol property, BoundExpression arg0)
         {
             Debug.Assert((receiverOpt == null) == property.IsStatic);
             return Call(receiverOpt, property.GetMethod, arg0); // TODO: should we use property.GetBaseProperty().GetMethod to ensure we generate a call to the overridden method?

src/Compilers/CSharp/Test/Emit/CodeGen/IndexAndRangeTests.cs

@@ -8,6 +8,22 @@ namespace Microsoft.CodeAnalysis.CSharp.UnitTests.CodeGen
 {
     public class IndexAndRangeTests : CSharpTestBase
     {
+        [Fact]
+        public void RangeIndexerStringFromEndStart()
+        {
+            var comp = CreateCompilationWithIndexAndRange(@"
+using System;
+class C
+{
+    public static void Main()
+    {
+        var s = ""abcdef"";
+        Console.WriteLine(s[^2..]);
+    }
+}", options: TestOptions.ReleaseExe);
+            CompileAndVerify(comp, expectedOutput: "ef");
+        }
+
         [Fact]
         public void IndexIndexerStringTwoArgs()
         {
@@ -480,6 +496,30 @@ .maxstack  5
 ");
         }
 
+        [Fact]
+        public void FakeRangeStartFromEndIndexerArray()
+        {
+            var comp = CreateCompilationWithIndexAndRange(@"
+using System;
+class C
+{
+    public static void Main()
+    {
+        var arr = new[] { 1, 2, 3, 11 };
+        var result = M(arr);
+        Console.WriteLine(result.Length);
+        foreach (var x in result)
+        {
+            Console.WriteLine(x);
+        }
+    }
+    public static int[] M(int[] array) => array[^2..];
+}", TestOptions.ReleaseExe);
+            CompileAndVerify(comp, expectedOutput: @"2
+3
+11");
+        }
+
         [Fact]
         public void FakeRangeToEndIndexerArray()
         {

src/Compilers/CSharp/Test/Symbol/Symbols/MissingSpecialMember.cs

@@ -884,6 +884,10 @@ public void AllWellKnownTypeMembers()
                     case WellKnownMember.System_ReadOnlySpan_T__get_Item:
                     case WellKnownMember.System_ReadOnlySpan_T__get_Length:
                     case WellKnownMember.System_Index__ctor:
+                    case WellKnownMember.System_Index__FromEnd:
+                    case WellKnownMember.System_Index__Value:
+                    case WellKnownMember.System_Range__Start:
+                    case WellKnownMember.System_Range__End:
                     case WellKnownMember.System_Range__All:
                     case WellKnownMember.System_Range__Create:
                     case WellKnownMember.System_Range__FromStart:

src/Compilers/VisualBasic/Test/Symbol/SymbolsTests/WellKnownTypeValidationTests.vb

@@ -657,6 +657,10 @@ End Namespace
                          WellKnownMember.System_Runtime_CompilerServices_IsByRefLikeAttribute__ctor,
                          WellKnownMember.System_Runtime_CompilerServices_IsUnmanagedAttribute__ctor,
                          WellKnownMember.System_Index__ctor,
+                         WellKnownMember.System_Index__FromEnd,
+                         WellKnownMember.System_Index__Value,
+                         WellKnownMember.System_Range__Start,
+                         WellKnownMember.System_Range__End,
                          WellKnownMember.System_Range__All,
                          WellKnownMember.System_Range__Create,
                          WellKnownMember.System_Range__FromStart,
@@ -779,6 +783,10 @@ End Namespace
                          WellKnownMember.System_Runtime_CompilerServices_IsByRefLikeAttribute__ctor,
                          WellKnownMember.System_Runtime_CompilerServices_IsUnmanagedAttribute__ctor,
                          WellKnownMember.System_Index__ctor,
+                         WellKnownMember.System_Index__FromEnd,
+                         WellKnownMember.System_Index__Value,
+                         WellKnownMember.System_Range__Start,
+                         WellKnownMember.System_Range__End,
                          WellKnownMember.System_Range__All,
                          WellKnownMember.System_Range__Create,
                          WellKnownMember.System_Range__FromStart,