Add articles for remaining C# 13 and C# 14 errors (#46376)

* Add target for remaining C# 13 errors

Add all errors and warnings for any warnings added in C# 13.

Also, add list of undocumented (as of yet) C# 14 errors.

* Add C# 14 error message before extensions

* consolidate existing extension articles

Create a new article to include all the existing extension errors and warnings.

* Add placeholders for all new extension errors

Add the description and fixes for all new extension errors.

* final edit pass

* fix warnings.

Author: BillWagner

.openpublishing.redirection.csharp.json

@@ -343,6 +343,10 @@
             "source_path_from_root": "/docs/csharp/language-reference/compiler-messages/cs0260.md",
             "redirect_url": "/dotnet/csharp/language-reference/compiler-messages/partial-declarations"
         },
+        {
+            "source_path_from_root": "/docs/csharp/language-reference/compiler-messages/cs1112.md",
+            "redirect_url": "/dotnet/csharp/language-reference/compiler-messages/extension-declarations"
+        },
         {
             "source_path_from_root": "/docs/csharp/language-reference/compiler-messages/cs1683.md",
             "redirect_url": "/dotnet/csharp/language-reference/compiler-messages/assembly-references"
@@ -1818,14 +1822,50 @@
             "source_path_from_root": "/docs/csharp/misc/cs1020.md",
             "redirect_url": "/dotnet/csharp/language-reference/compiler-messages/overload-resolution"
         },
+        {
+            "source_path_from_root": "/docs/csharp/misc/cs1100.md",
+            "redirect_url": "/dotnet/csharp/language-reference/compiler-messages/extension-declarations"
+        },
+        {
+            "source_path_from_root": "/docs/csharp/misc/cs1101.md",
+            "redirect_url": "/dotnet/csharp/language-reference/compiler-messages/extension-declarations"
+        },
+        {
+            "source_path_from_root": "/docs/csharp/misc/cs1102.md",
+            "redirect_url": "/dotnet/csharp/language-reference/compiler-messages/extension-declarations"
+        },
+        {
+            "source_path_from_root": "/docs/csharp/misc/cs1103.md",
+            "redirect_url": "/dotnet/csharp/language-reference/compiler-messages/extension-declarations"
+        },
         {
             "source_path_from_root": "/docs/csharp/misc/cs1104.md",
             "redirect_url": "/dotnet/csharp/language-reference/compiler-messages/params-arrays"
         },
+        {
+            "source_path_from_root": "/docs/csharp/misc/cs1105.md",
+            "redirect_url": "/dotnet/csharp/language-reference/compiler-messages/extension-declarations"
+        },
+        {
+            "source_path_from_root": "/docs/csharp/misc/cs1106.md",
+            "redirect_url": "/dotnet/csharp/language-reference/compiler-messages/extension-declarations"
+        },
         {
             "source_path_from_root": "/docs/csharp/misc/cs1007.md",
             "redirect_url": "/dotnet/csharp/language-reference/compiler-messages/overload-resolution"
         },
+        {
+            "source_path_from_root": "/docs/csharp/misc/cs1109.md",
+            "redirect_url": "/dotnet/csharp/language-reference/compiler-messages/extension-declarations"
+        },
+        {
+            "source_path_from_root": "/docs/csharp/misc/cs1110.md",
+            "redirect_url": "/dotnet/csharp/language-reference/compiler-messages/extension-declarations"
+        },
+        {
+            "source_path_from_root": "/docs/csharp/misc/cs1113.md",
+            "redirect_url": "/dotnet/csharp/language-reference/compiler-messages/extension-declarations"
+        },
         {
             "source_path_from_root": "/docs/csharp/language-reference/compiler-messages/cs1502.md",
             "redirect_url": "/dotnet/csharp/misc/cs1503"

docs/csharp/language-reference/compiler-messages/assembly-references.md

@@ -20,6 +20,7 @@ f1_keywords:
  - "CS7079"
  - "CS8090"
  - "CS8203"
+ - "CS9286"
 helpviewer_keywords:
  - "CS0012"
  - "CS0234"
@@ -39,7 +40,8 @@ helpviewer_keywords:
  - "CS7079"
  - "CS8090"
  - "CS8203"
-ms.date: 02/28/2024
+ - "CS9286"
+ms.date: 05/27/2025
 ---
 # Resolve errors and warnings related to assembly references
 
@@ -64,6 +66,7 @@ That's by design. The text closely matches the text of the compiler error / warn
 - [**CS7079**](#invalid-assembly-reference): *The type is defined in a module that has not been added. You must add the module.*
 - [**CS8090**](#invalid-assembly-reference): *There is an error in a referenced assembly.*
 - [**CS8203**](#invalid-assembly-reference): *Invalid assembly name.*
+- [**CS9286**](#missing-references): *Type does not contain a definition and no accessible extension member for receiver type could be found (are you missing a using directive or an assembly reference?)*
 
 In addition, the following warnings are covered in this article:
 
@@ -76,13 +79,15 @@ The following errors and warnings indicate that you're missing an assembly refer
 - **CS0012**: *The type 'type' is defined in an assembly that is not referenced. You must add a reference to assembly 'assembly'.*
 - **CS0234**: *The type or namespace name does not exist in the namespace (are you missing an assembly reference?)*
 - **CS0246**: *The type or namespace name could not be found (are you missing a using directive or an assembly reference?)*
+- **CS9286**: *Type does not contain a definition and no accessible extension member for receiver type could be found (are you missing a using directive or an assembly reference?)*
 
 These compiler errors indicate one of these problems in your code:
 
 - The project doesn't reference the required assembly. To fix this error, [add a reference to the required assembly](../../../standard/assembly/index.md#add-a-reference-to-an-assembly).
 - You misspelled the name of a type. Check the name of the type.
 - You used a variable name where the name of a <xref:System.Type?displayProperty=nameWithType> was expected, such as in the [`typeof` operator](../operators/type-testing-and-cast.md#the-typeof-operator) or the [`is` operator](../operators/type-testing-and-cast.md#the-is-operator).
 - You used the [global scope operator, (`::`)](../operators/namespace-alias-qualifier.md) when the type isn't in the global namespace.
+- You're accessing an extension member and either the namespace isn't specified in a `using` directive, or you're not referencing the assembly that contains the extension.
 
 ## Type forwarding
 

docs/csharp/language-reference/compiler-messages/cs1112.md

@@ -47,6 +47,8 @@ f1_keywords:
  - "CS9170"
  - "CS9175"
  - "CS9226"
+ - "CS9296"
+ - "CS9307"
 helpviewer_keywords:
  - "CS0765"
  - "CS0831"
@@ -94,7 +96,9 @@ helpviewer_keywords:
  - "CS9170"
  - "CS9175"
  - "CS9226"
-ms.date: 09/06/2023
+ - "CS9296"
+ - "CS9307"
+ms.date: 05/27/2025
 ---
 # Resolve errors and warnings generated from expressions prohibited in expression trees
 
@@ -149,6 +153,8 @@ That's by design. The text closely matches the text of the compiler error / warn
 - **CS9170** - *An expression tree may not contain an inline array access or conversion.*
 - **CS9175** - *An expression tree may not contain a collection expression.*
 - **CS9226** - *An expression tree may not contain an expanded form of non-array params collection parameter.*
+- **CS9296** - *An expression tree may not contain an extension property access*.
+- **CS9307** - *An expression tree may not contain a named argument specification out of position*.
 
 ## Expression tree restrictions
 
@@ -184,6 +190,7 @@ The following expressions are prohibited:
 
 Other restrictions are:
 
+- Extension properties can't be accessed as extensions.
 - Attributes can't be applied to the lambda expression, its parameters or return.
 - The lambda expression must be convertible to a type derived from <xref:System.Linq.Expressions.Expression?displayProperty=fullName> whose type parameter is a delegate type.
 - [named and optional parameters](../../programming-guide/classes-and-structs/named-and-optional-arguments.md) are restricted. The expression can't call a method specifying named arguments, and it can't use the default value of an optional parameter.