Fix IntroduceConstant to place fields outside extension blocks

src/Features/CSharp/Portable/IntroduceVariable/CSharpIntroduceVariableService_IntroduceField.cs

@@ -5,6 +5,7 @@
 #nullable disable
 
 using System;
+using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.CodeActions;
@@ -28,7 +29,13 @@ protected override Task<Document> IntroduceFieldAsync(
         bool isConstant,
         CancellationToken cancellationToken)
     {
-        var oldTypeDeclaration = expression.GetAncestorOrThis<TypeDeclarationSyntax>();
+        // Get the ancestor TypeDeclarationSyntax that is NOT an ExtensionBlockDeclarationSyntax.
+        // Extension blocks can't contain fields, so we need to find the containing class/struct.
+        //
+        // Note, this can be revised in the future as we do expect to allow constants/static in
+        // extension blocks in a future version of the language.
+        var oldTypeDeclaration = expression.GetAncestorsOrThis<TypeDeclarationSyntax>()
+            .FirstOrDefault(t => t is not ExtensionBlockDeclarationSyntax);
 
         var oldType = oldTypeDeclaration != null
             ? document.SemanticModel.GetDeclaredSymbol(oldTypeDeclaration, cancellationToken)

src/Features/CSharpTest/IntroduceVariable/IntroduceVariableTests.cs

@@ -8559,4 +8559,64 @@ public Customer* D
                 }
             }
             """);
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/81337")]
+    public Task TestExtensionBlockIntroduceConstant()
+        => TestInRegularAndScriptAsync(
+            """
+            public static class CEx
+            {
+                extension(C c)
+                {
+                    public string P => c[[|"P"|]];
+                }
+            }
+
+            public class C { public string this[string k] => ""; }
+            """,
+            """
+            public static class CEx
+            {
+                private const string {|Rename:V|} = "P";
+
+                extension(C c)
+                {
+                    public string P => c[V];
+                }
+            }
+
+            public class C { public string this[string k] => ""; }
+            """,
+            new(parseOptions: TestOptions.Regular.WithLanguageVersion(LanguageVersion.CSharp14)));
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/81337")]
+    public Task TestExtensionBlockIntroduceConstant_AllOccurrences()
+        => TestInRegularAndScriptAsync(
+            """
+            public static class CEx
+            {
+                extension(C c)
+                {
+                    public string P => c[[|"P"|]];
+                    public string P2 => c["P"];
+                }
+            }
+
+            public class C { public string this[string k] => ""; }
+            """,
+            """
+            public static class CEx
+            {
+                private const string {|Rename:V|} = "P";
+
+                extension(C c)
+                {
+                    public string P => c[V];
+                    public string P2 => c[V];
+                }
+            }
+
+            public class C { public string this[string k] => ""; }
+            """,
+            new(parseOptions: TestOptions.Regular.WithLanguageVersion(LanguageVersion.CSharp14), index: 1));
 }