updated conformance topic per product team

mikeblome · mikeblome · commit 2752fe08c282 · 2017-03-16T15:48:48.000-07:00
diff --git a/docs/cpp-conformance-improvements-2017.md b/docs/cpp-conformance-improvements-2017.md
@@ -276,6 +276,13 @@ constexpr bool test2 = !IsCallable<int*, int>::value;
 static_assert(test2, "PASS2");
 ```
 
+### Classes declared in anonymous namespaces
+According to the C++ standard, a class declared inside an anonymous namespace has internal linkage, and therefore cannot be exported. In Visual Studio 2015 and earlier, this rule was not enforced. In Visual Studio 2017 the rule is partially enforced. The following example raises this error in Visual Studio 2017: "error C2201: 'const `anonymous namespace'::S1::`vftable'': must have external linkage in order to be exported/imported."
+
+```cpp
+struct __declspec(dllexport) S1 { virtual void f() {} }; //C2201
+```
+
 ### Default initializers for value class members (C++/CLI)
 In Visual Studio 2015 and earlier, the compiler permitted (but ignored) a default member initializer for a member of a value class.  Default initialization of a value class always zero-initializes the members; a default constructor is not permitted.  In Visual Studio 2017, default member initializers raise a compiler error, as shown in this example:
 
