Add version check to enable the pattern-based Index & Range indexers (#35170)

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

@@ -7192,6 +7192,7 @@ private BoundExpression BindIndexerAccess(ExpressionSyntax node, BoundExpression
                     node,
                     expr,
                     analyzedArguments.Arguments,
+                    diagnostics,
                     out var patternIndexerAccess))
                 {
                     indexerAccessExpression = patternIndexerAccess;
@@ -7365,6 +7366,7 @@ private BoundExpression BindIndexedPropertyAccess(SyntaxNode syntax, BoundExpres
                         syntax,
                         receiverOpt,
                         analyzedArguments.Arguments,
+                        diagnostics,
                         out var patternIndexerAccess))
                     {
                         return patternIndexerAccess;
@@ -7464,6 +7466,7 @@ private BoundExpression BindIndexedPropertyAccess(SyntaxNode syntax, BoundExpres
             SyntaxNode syntax,
             BoundExpression receiverOpt,
             ArrayBuilder<BoundExpression> arguments,
+            DiagnosticBag diagnostics,
             out BoundIndexOrRangePatternIndexerAccess patternIndexerAccess)
         {
             // Verify a few things up-front, namely that we have a single argument
@@ -7542,6 +7545,7 @@ private BoundExpression BindIndexedPropertyAccess(SyntaxNode syntax, BoundExpres
                             property.OriginalDefinition is { ParameterCount: 1 } original &&
                             isIntNotByRef(original.Parameters[0]))
                         {
+                            _ = MessageID.IDS_FeatureIndexOperator.CheckFeatureAvailability(diagnostics, syntax.Location);
                             patternIndexerAccess = new BoundIndexOrRangePatternIndexerAccess(
                                 syntax,
                                 receiverOpt,
@@ -7562,6 +7566,7 @@ private BoundExpression BindIndexedPropertyAccess(SyntaxNode syntax, BoundExpres
                 var substring = (MethodSymbol)Compilation.GetSpecialTypeMember(SpecialMember.System_String__Substring);
                 if (substring is { })
                 {
+                    _ = MessageID.IDS_FeatureIndexOperator.CheckFeatureAvailability(diagnostics, syntax.Location);
                     patternIndexerAccess = new BoundIndexOrRangePatternIndexerAccess(
                         syntax,
                         receiverOpt,
@@ -7601,6 +7606,7 @@ private BoundExpression BindIndexedPropertyAccess(SyntaxNode syntax, BoundExpres
                             isIntNotByRef(original.Parameters[0]) &&
                             isIntNotByRef(original.Parameters[1]))
                         {
+                            _ = MessageID.IDS_FeatureIndexOperator.CheckFeatureAvailability(diagnostics, syntax.Location);
                             patternIndexerAccess = new BoundIndexOrRangePatternIndexerAccess(
                                 syntax,
                                 receiverOpt,

src/Compilers/CSharp/Test/Semantic/Semantics/IndexAndRangeTests.cs

@@ -15,6 +15,40 @@ public class IndexAndRangeTests : CompilingTestBase
         private const string RangeEndAtSignature = "System.Range System.Range.EndAt(System.Index end)";
         private const string RangeAllSignature = "System.Range System.Range.All.get";
 
+        [Fact]
+        public void PatternIndexRangeLangVer()
+        {
+            var src = @"
+using System;
+struct S
+{
+    public int Length => 0;
+    public int Slice(int x, int y) => 0;
+}
+class C
+{
+    void M(string s, Index i, Range r)
+    {
+        _ = s[i];
+        _ = s[r];
+        _ = new S()[r];
+    }
+}";
+            var comp = CreateCompilationWithIndexAndRange(src);
+            comp.VerifyDiagnostics();
+            comp = CreateCompilationWithIndexAndRange(src, parseOptions: TestOptions.Regular7_3);
+            comp.VerifyDiagnostics(
+                // (12,13): error CS8652: The feature 'index operator' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         _ = s[i];
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "s[i]").WithArguments("index operator").WithLocation(12, 13),
+                // (13,13): error CS8652: The feature 'index operator' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         _ = s[r];
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "s[r]").WithArguments("index operator").WithLocation(13, 13),
+                // (14,13): error CS8652: The feature 'index operator' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //         _ = new S()[r];
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "new S()[r]").WithArguments("index operator").WithLocation(14, 13));
+        }
+
         [Fact]
         public void SpanPatternRangeDelegate()
         {