Explicitly delete some Callback constructors to improve errors.

base/callback_internal.h

Index: base/callback_internal.h
diff --git a/base/callback_internal.h b/base/callback_internal.h
index f7501f96c5d737f0a0be70590743c45e6b05329a..0b70750726ca5247ed07b7ce993659e78ef18518 100644
--- a/base/callback_internal.h
+++ b/base/callback_internal.h
@@ -79,6 +79,9 @@ class BASE_EXPORT CallbackBase<CopyMode::MoveOnly> {
   CallbackBase(CallbackBase&& c);
   CallbackBase& operator=(CallbackBase&& c);
 
+  CallbackBase(const CallbackBase&) = delete;
+  CallbackBase& operator=(const CallbackBase&) = delete;

[dcheng, 2016/12/19 11:07:40]

Deleting it in the Callback template helper itself would improve the error even
further, but it's tricky to do this:
http://stackoverflow.com/questions/27073082/conditionally-disabling-a-copy-co...

I'm not even sure it's possible to apply that pattern here, as it defines a
inheritance relation between the partial specialization and the base template...
but the Callback template already inherits from other things.

[brucedawson, 2016/12/19 19:16:00]

Simple is probably better in that case.

+
   explicit CallbackBase(const CallbackBase<CopyMode::Copyable>& c);
   CallbackBase& operator=(const CallbackBase<CopyMode::Copyable>& c);