Clarify struct usage guidelines

The current coding standards restrict the use of struct to PODs, but no
one has been following them.  This patch updates the standards to
clarify when structs are dangerous and describe common practice in LLVM.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202728 91177308-0d34-0410-b5e6-96231b3b80d8

docs/CodingStandards.rst

@@ -650,12 +650,38 @@ members public by default.
 
 Unfortunately, not all compilers follow the rules and some will generate
 different symbols based on whether ``class`` or ``struct`` was used to declare
-the symbol.  This can lead to problems at link time.
+the symbol (e.g., MSVC).  This can lead to problems at link time.
 
-So, the rule for LLVM is to always use the ``class`` keyword, unless **all**
-members are public and the type is a C++ `POD
-<http://en.wikipedia.org/wiki/Plain_old_data_structure>`_ type, in which case
-``struct`` is allowed.
+* All declarations and definitions of a given ``class`` or ``struct`` must use
+  the same keyword.  For example:
+
+.. code-block:: c++
+
+  class Foo;
+
+  // Breaks mangling in MSVC.
+  struct Foo { int Data; };
+
+* As a rule of thumb, ``struct`` should be kept to structures where *all*
+  members are declared public.
+
+.. code-block:: c++
+
+  // Foo feels like a class... this is strange.
+  struct Foo {
+  private:
+    int Data;
+  public:
+    Foo() : Data(0) { }
+    int getData() const { return Data; }
+    void setData(int D) { Data = D; }
+  };
+
+  // Bar isn't POD, but it does look like a struct.
+  struct Bar {
+    int Data;
+    Foo() : Data(0) { }
+  };
 
 Do not use Braced Initializer Lists to Call a Constructor
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^