Add compiler diagnostics related to file scoped types

docs/csharp/language-reference/compiler-messages/file-local-types.md

@@ -0,0 +1,82 @@
+---
+title: Resolve errors and warnings in file-local types
+description: Learn how to diagnose and correct C# compiler errors and warnings when you declare file local types, or write source generators that create file local types.
+f1_keywords:
+  - "CS9051"
+  - "CS9052"
+  - "CS9053"
+  - "CS9054"
+  - "CS9055"
+  - "CS9056"
+  - "CS9068"
+  - "CS9069"
+  - "CS9071"
+helpviewer_keywords:
+  - "CS9051"
+  - "CS9052"
+  - "CS9053"
+  - "CS9054"
+  - "CS9055"
+  - "CS9056"
+  - "CS9068"
+  - "CS9069"
+  - "CS9071"
+ms.date: 01/13/2026
+ai-usage: ai-assisted
+---
+# Resolve errors and warnings for file local types
+
+The C# compiler generates errors and warnings when you misuse [file-local types](../keywords/file.md). File-local types are visible only within the file where you declare them. These diagnostics help you follow the rules for declaring and using file-local types.
+
+<!-- The text in this list generates issues for Acrolinx, because they don't use contractions.
+That's by design. The text closely matches the text of the compiler error / warning for SEO purposes.
+ -->
+- [**CS9051**](#visibility-restrictions): *File-local type cannot be used in a member signature in non-file-local type.*
+- [**CS9052**](#visibility-restrictions): *File-local type cannot use accessibility modifiers.*
+- [**CS9053**](#visibility-restrictions): *File-local type cannot be used as a base type of non-file-local type.*
+- [**CS9054**](#declaration-rules): *File-local type must be defined in a top level type; it is a nested type.*
+- [**CS9055**](#visibility-restrictions): *File-local type cannot be used in a 'global using static' directive.*
+- [**CS9056**](#declaration-rules): *Types and aliases cannot be named 'file'.*
+- [**CS9068**](#file-path-requirements): *File-local type must be declared in a file with a unique path. Path is used in multiple files.*
+- [**CS9069**](#file-path-requirements): *File-local type cannot be used because the containing file path cannot be converted into the equivalent UTF-8 byte representation.*
+- [**CS9071**](#declaration-rules): *The namespace already contains a definition for the type in this file.*
+
+## Visibility restrictions
+
+- **CS9051**: *File-local type cannot be used in a member signature in non-file-local type.*
+- **CS9052**: *File-local type cannot use accessibility modifiers.*
+- **CS9053**: *File-local type cannot be used as a base type of non-file-local type.*
+- **CS9055**: *File-local type cannot be used in a 'global using static' directive.*
+
+A [file-local type](../keywords/file.md) is visible only within the file containing its declaration. The compiler enforces restrictions to prevent file-local types from "leaking" outside their intended scope:
+
+- A file-local type can't appear in the signature (parameters, return type, or type constraints) of any member declared in a non-file-local type (**CS9051**). This restriction ensures that code in other files doesn't depend on types it can't access.
+- File-local types can't have explicit accessibility modifiers like `public`, `internal`, or `private` (**CS9052**). The `file` modifier already defines the type's visibility scope, making other access modifiers meaningless.
+- A non-file-local type can't inherit from a file-local type (**CS9053**). If a derived type is visible outside the file, its base type must also be visible. However, a non-file-local type *can* implement a file-local interface.
+- A file-local type can't be used in a `global using static` directive (**CS9055**). Global usings apply to all files in the compilation, but file-local types are visible only in their declaring file.
+
+To resolve these errors, either remove the `file` modifier from the type to make it accessible outside the file, or change the consuming code to avoid exposing the file-local type.
+
+## Declaration rules
+
+- **CS9054**: *File-local type must be defined in a top level type; it is a nested type.*
+- **CS9056**: *Types and aliases cannot be named 'file'.*
+- **CS9071**: *The namespace already contains a definition for the type in this file.*
+
+The compiler enforces rules about how file-local types are declared:
+
+- File-local types must be declared at the top level of a file, not nested within another type (**CS9054**). To resolve this error, move the type declaration outside any containing type, or remove the `file` modifier.
+- Types and type aliases can't be named `file` because it's now a contextual keyword (**CS9056**). To resolve this error, rename the type or use `@file` to escape the identifier.
+- A file can't declare multiple file-local types with the same name in the same namespace (**CS9071**). Each file-local type must have a unique name within its namespace scope in that file. To resolve this error, rename one of the conflicting types.
+
+## File path requirements
+
+- **CS9068**: *File-local type must be declared in a file with a unique path. Path is used in multiple files.*
+- **CS9069**: *File-local type cannot be used because the containing file path cannot be converted into the equivalent UTF-8 byte representation.*
+
+The compiler uses the file path to generate unique internal names for file-local types. This naming scheme enables multiple files to declare file-local types with the same name without conflict:
+
+- Each file containing file-local types must have a unique path within the compilation (**CS9068**). This error typically occurs with source generators that produce multiple files with the same path. To resolve this error, ensure each generated file has a distinct path.
+- The file path must be convertible to UTF-8 encoding (**CS9069**). This error occurs when the file path contains characters that can't be represented in UTF-8. To resolve this error, rename the file or directory to use characters that are valid in UTF-8.
+
+These errors most commonly affect source generator authors. For more information about using file-local types in source generators, see [file-local types](../keywords/file.md).

docs/csharp/language-reference/compiler-messages/using-directive-errors.md

@@ -20,7 +20,6 @@ f1_keywords:
   - "CS8914"
   - "CS8915"
   - "CS8933"
-  - "CS9055"
   - "CS9130"
   - "CS9131"
   - "CS9132"
@@ -45,7 +44,6 @@ helpviewer_keywords:
   - "CS8914"
   - "CS8915"
   - "CS8933"
-  - "CS9055"
   - "CS9130"
   - "CS9131"
   - "CS9132"
@@ -73,7 +71,6 @@ That's be design. The text closely matches the text of the compiler error / warn
 - [**CS8085**](#restrictions-on-using-aliases): *Error: A 'using static' directive cannot be used to declare an alias.*
 - [**CS8914**](#global-using-directive): *Error: A global using directive cannot be used in a namespace declaration.*
 - [**CS8915**](#global-using-directive): *Error: A global using directive must precede all non-global using directives.*
-- [**CS9055**](#global-using-directive): *Error: A file-local type cannot be used in a 'global using static' directive.*
 - [**CS9130**](#restrictions-on-using-aliases): *Error: Using alias cannot be a `ref` type.*
 - [**CS9131**](#restrictions-on-using-aliases): *Error: Only a using alias can be `unsafe`.*
 - [**CS9132**](#restrictions-on-using-aliases): *Error: Using alias cannot be a nullable reference type.*

docs/csharp/language-reference/toc.yml

@@ -644,7 +644,7 @@ items:
     href: ./compiler-messages/using-directive-errors.md
     displayName: >
       CS0105, CS0138, CS0431, CS0432, CS0440, CS0576, CS0687, CS1529, CS1537, CS7000,
-      CS7007, CS8019, CS8083, CS8085, CS8914, CS8915, CS8933, CS9055, CS9130, CS9131,
+      CS7007, CS8019, CS8083, CS8085, CS8914, CS8915, CS8933, CS9130, CS9131,
       CS9132, CS9133, CS9162, CS9163
   - name: Using statements and declarations
     href: ./compiler-messages/using-statement-declaration-errors.md
@@ -657,6 +657,10 @@ items:
       CS9147, CS9148, CS9149, CS9150, CS9151, CS9152, CS9153, CS9154, CS9155, CS9156,
       CS9157, CS9158, CS9159, CS9160, CS9161, CS9177, CS9178, CS9206, CS9207, CS9231,
       CS9232, CS9233, CS9234, CS9235, CS9270
+  - name: File local types
+    href: ./compiler-messages/file-local-types.md
+    displayName: >
+      CS9051, CS9052, CS9053, CS9054, CS9055, CS9056, CS9068, CS9069, CS9071
   - name: static abstract interface members
     href: ./compiler-messages/static-abstract-interfaces.md
     displayName: >

docs/csharp/misc/sorry-we-don-t-have-specifics-on-this-csharp-error.md

@@ -457,17 +457,8 @@ f1_keywords:
   - "CS9044"
   - "CS9046"
   - "CS9049"
-  - "CS9051"
-  - "CS9052"
-  - "CS9053"
-  - "CS9054"
-  - "CS9056"
-  - "CS9058"
   - "CS9064"
   - "CS9067"
-  - "CS9068"
-  - "CS9070"
-  - "CS9071"
 helpviewer_keywords:
   - "errors [C#], additional information"
 ---