Adds ScopedHandleBase::From

mojo/public/cpp/system/core.h

Index: mojo/public/cpp/system/core.h
diff --git a/mojo/public/cpp/system/core.h b/mojo/public/cpp/system/core.h
index 70221107ab0a2d6ee332d461294840c34c601d55..fc65ea5cc6fe386c13aa4d8ba510654423d222ab 100644
--- a/mojo/public/cpp/system/core.h
+++ b/mojo/public/cpp/system/core.h
@@ -49,6 +49,16 @@ class ScopedHandleBase {
 
   const HandleType& get() const { return handle_; }
 
+  template <typename PassedHandleType>
+  static ScopedHandleBase<HandleType> From(
+      ScopedHandleBase<PassedHandleType> other) {
+    MOJO_COMPILE_ASSERT(
+        sizeof(static_cast<PassedHandleType*>(static_cast<HandleType*>(0))),
+        HandleType_is_not_a_subtype_of_PassedHandleType);
+    return ScopedHandleBase<HandleType>(
+        static_cast<HandleType>(other.release().value()));

[darin (slow to review), 2014/04/04 22:40:49]

Are you sure about the .value() call here? Doesn't that strip the
PassedHandleType down to a MojoHandle, which any HandleType can be initialized
from? It feels like this might allow you to initialize a
ScopedHandleBase<MessagePipeHandle> from a
ScopedHandleBase<DataPipeConsumerHandle>.

I would have expected this to work out if you leave off the .value() call.

[darin (slow to review), 2014/04/04 22:56:14]

Nevermind. I somehow missed the compile assert :-P

+  }
+
   void swap(ScopedHandleBase& other) {
     handle_.swap(other.handle_);
   }