[PATCH] [libcxx] LWG2442: call_once() shouldn't DECAY_COPY()

[Eric Fiselier]

================
Comment at: include/mutex:445
@@ -444,3 +444,3 @@
 _LIBCPP_INLINE_VISIBILITY
-void call_once(once_flag&, _Callable);
 
----------------
This change is being made in order to support non-copyable Callable objects, right? I'm not convinced that always adding const is the correct thing to do. Without the const the new signature won't accept rvalues as input but adding const requires Callable to be const-callable.

Why not provide two signatures? It's not perfect but I think it is better.
```
void call_once(once_flag&, _Callable&); // accepts non-const lvalues.
void call_once(once_flag&, _Callable const&); // accepts rvalues and const lvalues.
```

Thoughts?



================
Comment at: test/std/thread/thread.mutex/thread.once/thread.once.callonce/call_once.pass.cpp:160
@@ +159,3 @@
+   // GCC 4.8 complains about the following being private
+public:
+    NonCopyable(const NonCopyable&)
----------------
That seems like it might indicate a problem. Do you know what is going on here? Are you sure you want to work around this?

================
Comment at: test/std/thread/thread.mutex/thread.once/thread.once.callonce/call_once.pass.cpp:224
@@ -206,1 +223,3 @@
     }
+    // check LWG2442: call_once() shouldn't DECAY_COPY()
+    {
----------------
Could you also add tests for the following:
1. We don't copy the function.
2. We accept a temporary as a function.
3. If given a Function object with `operator()(...) &` and `operator()(...) &&` do we call the correct one depending on the value category of the given functor?

http://reviews.llvm.org/D10191

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