Merge pull request #2999 from corob-msft/docs/corob/cpp-docs-2313-C5208

Address 2313 Add warning C5208

Author: ttorble

docs/error-messages/compiler-warnings/c5208.md

@@ -0,0 +1,65 @@
+---
+title: "Compiler Warning C5208, Error C7626"
+description: Compiler warning C5208 description and solution.
+ms.date: 07/17/2020
+f1_keywords: ["C5208", "C7626"]
+helpviewer_keywords: ["C5208", "C7626"]
+---
+# Compiler Warning (level 1) C5208 and Error C7626
+
+> unnamed class used in typedef name cannot declare members other than non-static data members, member enumerations, or member classes
+
+## Remarks
+
+Unnamed classes within a **`typedef`** declaration can't have any members other than:
+
+- non-static data members,
+- member classes,
+- member enumerations,
+- and default member initializers.
+
+The same restrictions are applied recursively to each nested class. The restriction is meant to ensure the simplicity of structs that have `typedef` names for linkage purposes. They must be simple enough that no linkage calculations are necessary before the compiler gets to the `typedef` name for linkage.
+
+This warning is new in Visual Studio 2019 version 16.6. Based on [P1766R1](https://wg21.link/P1766R1) adopted by the C++ standards committee as a defect report, it affects all standards modes of the compiler. In default **`/std:c++14`** and **`/std:c++17`** modes, the compiler emits warning C5208 for non-conforming code. If **`/permissive-`** is specified, the compiler emits warning C5208 as an error under **`/std:c++14`** and emits error C7626 under **`/std:c++17`**. The compiler emits error C7626 for non-conforming code when **`/std:c++latest`** is specified.
+
+### To turn off the warning without code changes
+
+You can turn off the warning for a specific line of code by using the [warning](../../preprocessor/warning.md) pragma, `#pragma warning(suppress : 5208)`. You can also turn off the warning within a file by using the warning pragma, `#pragma warning(disable : 5208)`. You can turn off the warning globally in command-line builds by using the **/wd5208** command-line option.
+
+To turn off the warning for an entire project in the Visual Studio IDE:
+
+1. Open the **Property Pages** dialog for your project. For information on how to use the Property Pages dialog, see [Property Pages](../../build/reference/property-pages-visual-cpp.md).
+1. Select the **Configuration Properties** > **C/C++** > **Advanced** page.
+1. Edit the **Disable Specific Warnings** property to add *`5208`*. Choose **OK** to apply your changes.
+
+## Example
+
+The following sample shows the constructs that are no longer allowed in unnamed structs. Depending on the standards mode specified, C5208 or C7626 errors or warnings are emitted:
+
+```cpp
+struct Base { };
+typedef struct : Base // inheriting from 'Base'; ill-formed
+{
+ void fn(); // ill-formed
+ static int i; // ill-formed
+ struct U {
+  void f(); // nested class has non-data member; ill-formed
+ };
+ int j = 10; // default member initializer; ill-formed
+} S;
+```
+
+The code above can be fixed by giving the unnamed class a name:
+
+```cpp
+struct Base { };
+typedef struct NamedType : Base
+{
+ void fn();
+ static int i;
+ struct U {
+  void f();
+ };
+ int j = 10;
+} S;
+```