Fix #803: Use C# 9 relational patterns for VB Case Is <op> constant

When converting VB `Select Case` with `Case Is < value`, `Case Is > value`,
etc. (RelationalCaseClauseSyntax), emit clean C# 9 relational patterns
(`case < 1000:`) instead of the verbose `case var @case when @case < 1000:`
pattern, for non-object, non-string comparisons where the case value is a
constant expression.

Also handle RelationalPatternSyntax in the C#→VB converter so that C# 9
relational patterns round-trip correctly back to VB `Case Is <op> value`.

https://claude.ai/code/session_01AkwUvu3XuCdj3D4axoX4UX

Author: claude

CodeConverter/CSharp/MethodBodyExecutableStatementVisitor.cs

@@ -843,6 +843,13 @@ public override async Task<SyntaxList<StatementSyntax>> VisitSelectBlock(VBSynta
                     if (isObjectComparison) {
                         caseSwitchLabelSyntax = WrapInCasePatternSwitchLabelSyntax(node, relational.Value, csRelationalValue, false, operatorKind);
                     }
+                    else if (!isStringComparison && _semanticModel.GetConstantValue(relational.Value).HasValue) {
+                        csRelationalValue = CommonConversions.TypeConversionAnalyzer.AddExplicitConversion(relational.Value, csRelationalValue);
+                        var operatorToken = SyntaxFactory.Token(GetRelationalPatternTokenKind(operatorKind));
+                        caseSwitchLabelSyntax = SyntaxFactory.CasePatternSwitchLabel(
+                            SyntaxFactory.RelationalPattern(operatorToken, csRelationalValue),
+                            SyntaxFactory.Token(SyntaxKind.ColonToken));
+                    }
                     else {
                         var varName = CommonConversions.CsEscapedIdentifier(GetUniqueVariableNameInScope(node, "case"));
                         ExpressionSyntax csLeft = ValidSyntaxFactory.IdentifierName(varName);
@@ -1038,6 +1045,14 @@ private CasePatternSwitchLabelSyntax WrapInCasePatternSwitchLabelSyntax(VBSyntax
         _ => throw new ArgumentOutOfRangeException(nameof(caseClauseKind), caseClauseKind, null)
     };
 
+    private static CS.SyntaxKind GetRelationalPatternTokenKind(VBasic.SyntaxKind caseClauseKind) => caseClauseKind switch {
+        VBasic.SyntaxKind.CaseLessThanClause => CS.SyntaxKind.LessThanToken,
+        VBasic.SyntaxKind.CaseLessThanOrEqualClause => CS.SyntaxKind.LessThanEqualsToken,
+        VBasic.SyntaxKind.CaseGreaterThanOrEqualClause => CS.SyntaxKind.GreaterThanEqualsToken,
+        VBasic.SyntaxKind.CaseGreaterThanClause => CS.SyntaxKind.GreaterThanToken,
+        _ => throw new ArgumentOutOfRangeException(nameof(caseClauseKind), caseClauseKind, null)
+    };
+
     private ExpressionSyntax ComparisonAdjustedForStringComparison(VBSyntax.SelectBlockSyntax node, VBSyntax.ExpressionSyntax vbCase, TypeInfo lhsTypeInfo, ExpressionSyntax csLeft, ExpressionSyntax csRight, TypeInfo rhsTypeInfo, ComparisonKind comparisonKind)
     {
         var vbEquality = CommonConversions.VisualBasicEqualityComparison;

CodeConverter/VB/NodesVisitor.cs

@@ -1643,6 +1643,12 @@ public override VisualBasicSyntaxNode VisitThrowExpression(CSSyntax.ThrowExpress
 
     public override VisualBasicSyntaxNode VisitCasePatternSwitchLabel(CSSyntax.CasePatternSwitchLabelSyntax node)
     {
+        if (node.Pattern is CSSyntax.RelationalPatternSyntax relational) {
+            var value = (ExpressionSyntax)relational.Expression.Accept(TriviaConvertingVisitor);
+            var (caseClauseKind, tokenKind) = GetRelationalCaseClauseKinds(relational.OperatorToken.Kind());
+            return SyntaxFactory.RelationalCaseClause(caseClauseKind,
+                SyntaxFactory.Token(tokenKind), value);
+        }
         var condition = node.WhenClause.Condition.SkipIntoParens();
         switch (condition) {
             case CSSyntax.BinaryExpressionSyntax bes when node.Pattern.ToString().StartsWith("var", StringComparison.InvariantCulture): //VarPatternSyntax (not available in current library version)
@@ -1654,6 +1660,15 @@ public override VisualBasicSyntaxNode VisitCasePatternSwitchLabel(CSSyntax.CaseP
                 throw new NotSupportedException(condition.GetType() + " in switch case");
         }
     }
+
+    private static (SyntaxKind CaseClauseKind, SyntaxKind TokenKind) GetRelationalCaseClauseKinds(CS.SyntaxKind csTokenKind) => csTokenKind switch {
+        CS.SyntaxKind.LessThanToken => (SyntaxKind.CaseLessThanClause, SyntaxKind.LessThanToken),
+        CS.SyntaxKind.LessThanEqualsToken => (SyntaxKind.CaseLessThanOrEqualClause, SyntaxKind.LessThanEqualsToken),
+        CS.SyntaxKind.GreaterThanToken => (SyntaxKind.CaseGreaterThanClause, SyntaxKind.GreaterThanToken),
+        CS.SyntaxKind.GreaterThanEqualsToken => (SyntaxKind.CaseGreaterThanOrEqualClause, SyntaxKind.GreaterThanEqualsToken),
+        _ => throw new NotSupportedException($"Relational operator token {csTokenKind} not supported in VB case clause")
+    };
+
     private INamedTypeSymbol GetStructOrClassSymbol(CS.CSharpSyntaxNode node) {
         return (INamedTypeSymbol)_semanticModel.GetDeclaredSymbol(node.Ancestors().First(x => x is CSSyntax.ClassDeclarationSyntax || x is CSSyntax.StructDeclarationSyntax));
     }

Tests/CSharp/StatementTests/MethodStatementTests.cs

@@ -1089,13 +1089,13 @@ public static string TimeAgo(int daysAgo)
         {
             case var @case when 0 <= @case && @case <= 3:
             case 4:
-            case var case1 when case1 >= 5:
-            case var case2 when case2 < 6:
-            case var case3 when case3 <= 7:
+            case >= 5:
+            case < 6:
+            case <= 7:
                 {
                     return ""this week"";
                 }
-            case var case4 when case4 > 0:
+            case > 0:
                 {
                     return daysAgo / 7 + "" weeks ago"";
                 }
@@ -1292,15 +1292,15 @@ public void DoesNotThrow()
         var rand = new Random();
         switch (rand.Next(8))
         {
-            case var @case when @case < 4:
+            case < 4:
                 {
                     break;
                 }
             case 4:
                 {
                     break;
                 }
-            case var case1 when case1 > 4:
+            case > 4:
                 {
                     break;
                 }
@@ -1311,9 +1311,46 @@ public void DoesNotThrow()
                 }
         }
     }
-}
-1 target compilation errors:
-CS0825: The contextual keyword 'var' may only appear within a local variable declaration or in script code");
+}");
+    }
+
+    [Fact]
+    public async Task Issue803SelectCaseIsRelationalUsesC9PatternAsync()
+    {
+        await TestConversionVisualBasicToCSharpAsync(@"Public Class TestClass
+    Function Rollo(Breite As Integer) As Integer
+        Select Case Breite
+            Case Is < 1000
+                Return 12
+            Case Is < 1200
+                Return 15
+            Case Else
+                Return 28
+        End Select
+    End Function
+End Class", @"
+public partial class TestClass
+{
+    public int Rollo(int Breite)
+    {
+        switch (Breite)
+        {
+            case < 1000:
+                {
+                    return 12;
+                }
+            case < 1200:
+                {
+                    return 15;
+                }
+
+            default:
+                {
+                    return 28;
+                }
+        }
+    }
+}");
     }
 
     [Fact]