Publish extensions specification

Publish the feature specification for the extension members feature.

Author: BillWagner

docfx.json

@@ -59,7 +59,8 @@
                     "first-class-span-types.md",
                     "simple-lambda-parameters-with-modifiers.md",
                     "partial-events-and-constructors.md",
-                    "null-conditional-assignment.md"
+                    "null-conditional-assignment.md",
+                    "extensions.md"
                 ],
                 "src": "_csharplang/proposals",
                 "dest": "csharp/language-reference/proposals",
@@ -510,7 +511,7 @@
                 "_csharplang/proposals/csharp-11.0/*.md": "09/30/2022",
                 "_csharplang/proposals/csharp-12.0/*.md": "08/15/2023",
                 "_csharplang/proposals/csharp-13.0/*.md": "10/31/2024",
-                "_csharplang/proposals/*.md": "04/04/2025",
+                "_csharplang/proposals/*.md": "04/08/2025",
                 "_roslyn/docs/compilers/CSharp/Compiler Breaking Changes - DotNet 7.md": "11/08/2022",
                 "_roslyn/docs/compilers/CSharp/Compiler Breaking Changes - DotNet 8.md": "11/08/2023",
                 "_roslyn/docs/compilers/CSharp/Compiler Breaking Changes - DotNet 9.md": "11/09/2024",
@@ -691,6 +692,7 @@
                 "_csharplang/proposals/simple-lambda-parameters-with-modifiers.md": "Simple lambda parameters with modifiers",
                 "_csharplang/proposals/partial-events-and-constructors.md": "Partial events and constructors",
                 "_csharplang/proposals/null-conditional-assignment.md": "Null conditional assignment",
+                "_csharplang/proposals/extensions.md": "Extension members",
                 "_roslyn/docs/compilers/CSharp/Compiler Breaking Changes - DotNet 7.md": "C# compiler breaking changes since C# 10",
                 "_roslyn/docs/compilers/CSharp/Compiler Breaking Changes - DotNet 8.md": "C# compiler breaking changes since C# 11",
                 "_roslyn/docs/compilers/CSharp/Compiler Breaking Changes - DotNet 9.md": "C# compiler breaking changes since C# 12",
@@ -817,6 +819,7 @@
                 "_csharplang/proposals/simple-lambda-parameters-with-modifiers.md": "This proposal allows lambda parameters to be declared with modifiers without requiring their type names. You can add modifiers like `ref` and `out` to lambda parameters without specifying their type.",
                 "_csharplang/proposals/partial-events-and-constructors.md": "This proposal allows partial events and constructors to be declared in partial classes. The event and constructor can be split across class declarations.",
                 "_csharplang/proposals/null-conditional-assignment.md": "This proposal allows the null conditional operator to be used for the destination of assignment expressions. This allows you to assign a value to a property or field only if the left side is not null.",
+                "_csharplang/proposals/extensions.md": "This proposal enables new kinds of extension members. These new extension members support extension properties, extension static members, including extension operators.",
                 "_roslyn/docs/compilers/CSharp/Compiler Breaking Changes - DotNet 7.md": "Learn about any breaking changes since the initial release of C# 10 and included in C# 11",
                 "_roslyn/docs/compilers/CSharp/Compiler Breaking Changes - DotNet 8.md": "Learn about any breaking changes since the initial release of C# 11 and included in C# 12",
                 "_roslyn/docs/compilers/CSharp/Compiler Breaking Changes - DotNet 9.md": "Learn about any breaking changes since the initial release of C# 12 and included in C# 13",

docs/csharp/specification/toc.yml

@@ -219,6 +219,8 @@ items:
       href: ../../../_csharplang/proposals/csharp-10.0/async-method-builders.md
     - name: Params collections
       href: ../../../_csharplang/proposals/csharp-13.0/params-collections.md
+    - name: Extension members
+      href: ../../../_csharplang/proposals/extensions.md
   - name: Structs
     items:
     - name: Readonly instance members

docs/csharp/whats-new/csharp-14.md

@@ -8,6 +8,7 @@ ms.topic: whats-new
 
 C# 14 includes the following new features. You can try these features using the latest [Visual Studio 2022](https://visualstudio.microsoft.com/vs/preview/) version or the [.NET 10 SDK](https://dotnet.microsoft.com/download/dotnet):
 
+- [Extension members](#extension-members)
 - [Null-conditional assignment](#null-conditional-assignment)
 - [`nameof` supports unbound generic types](#unbound-generic-types-and-nameof)
 - [More implicit conversions for `Span<T>` and `ReadOnlySpan<T>`](#implicit-span-conversions)
@@ -25,6 +26,10 @@ You can find any breaking changes introduced in C# 14 in our article on [breakin
 
 [!INCLUDE [released-version-feedback](./includes/released-feedback.md)]
 
+## Extension members
+
+You can learn more details by reading the [feature specification](~/_csharplang/proposals/extensions.md) for the new extension members feature.
+
 ## The `field` keyword
 
 The token `field` enables you to write a property accessor body without declaring an explicit backing field. The token `field` is replaced with a compiler synthesized backing field.