Extensions parsing

docs/compilers/CSharp/Compiler Breaking Changes - DotNet 10.md

@@ -291,3 +291,29 @@ unsafe record struct R(
     public bool Equals(R other) => true;
 }
 ```
+
+## `extension` treated as a contextual keyword
+
+PROTOTYPE record which version this break is introduced in
+
+Starting with C# 14, the `extension` keyword serves a special purpose in denoting extension containers. 
+This changes how the compiler interprets certain code constructs.
+
+If you need to use "extension" as an identifier rather than a keyword, escape it with the `@` prefix: `@extension`. This tells the compiler to treat it as a regular identifier instead of a keyword.
+
+The compiler will parse this as an extension container rather than a constructor.
+```csharp
+class extension
+{
+    extension(object o) { } // parsed as an extension container
+}
+```
+
+The compiler will fail to parse this as a method with return type `extension`.
+```csharp
+class extension
+{
+    extension M() { } // will not compile
+}
+```
+

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -2600,7 +2600,7 @@ A catch() block after a catch (System.Exception e) block can catch non-CLS excep
     <value>Invalid preprocessor expression</value>
   </data>
   <data name="ERR_InvalidMemberDecl" xml:space="preserve">
-    <value>Invalid token '{0}' in class, record, struct, or interface member declaration</value>
+    <value>Invalid token '{0}' in class, record, struct, interface, or extension member declaration</value>
   </data>
   <data name="ERR_MemberNeedsType" xml:space="preserve">
     <value>Method must have a return type</value>
@@ -8047,4 +8047,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="WRN_InterceptsLocationAttributeUnsupportedSignature_Title" xml:space="preserve">
     <value>'InterceptsLocationAttribute(string, int, int)' is not supported. Move to 'InterceptableLocation'-based generation of these attributes instead. (https://github.com/dotnet/roslyn/issues/72133)</value>
   </data>
+  <data name="IDS_FeatureExtensions" xml:space="preserve">
+    <value>extensions</value>
+  </data>
 </root>

src/Compilers/CSharp/Portable/Errors/MessageID.cs

@@ -296,6 +296,8 @@ internal enum MessageID
         IDS_FeatureFirstClassSpan = MessageBase + 12849,
         IDS_FeatureUnboundGenericTypesInNameof = MessageBase + 12850,
         IDS_FeatureSimpleLambdaParameterModifiers = MessageBase + 12851,
+
+        IDS_FeatureExtensions = MessageBase + 12852,
     }
 
     // Message IDs may refer to strings that need to be localized.
@@ -480,6 +482,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
                 case MessageID.IDS_FeatureFirstClassSpan:
                 case MessageID.IDS_FeatureUnboundGenericTypesInNameof:
                 case MessageID.IDS_FeatureSimpleLambdaParameterModifiers:
+                case MessageID.IDS_FeatureExtensions:
                     return LanguageVersion.Preview;
 
                 // C# 13.0 features.