[libcxx] Rewrite C++03 __invoke.

Summary:
This patch rewrites the C++03 `__invoke` and related meta-programming. There are a number of major changes.

`__invoke` in C++03 now has a fallback overload for when the invoke expression is ill-formed (similar to C++11). This means that the `__invoke_return` traits will return `__nat` when `__invoke(...)` is ill formed. This would previously cause a compile error.

Bullets 1-4 of `__invoke` have been rewritten. In the old version `__invoke` had 32 overloads for bullets 1 and 2,
one for each possible cv-qualified function signature with arities 0-3. 64 overloads would be needed to support member functions
with varargs. Currently these overloads were fundamentally broken. An example overload looked like:
```
template <class Rp, class Tp, class T1, class A0>
Rp __invoke(Rp (Tp::*pm)(A0) const, T1&, A0&)
```
Because `A0` appeared in two different deducible contexts it would have to deduce to be an exact match or the overload
would be rejected. This is made even worse because `A0` appears without a reference qualifier in the member function signature
and with a reference qualifier as an `__invoke` parameter. This means that only member functions that took all
of their arguments by value could be matched.

One possible fix would be to make the second occurrence of `A0` appear in a non-deducible context. This way
any type convertible to `A0` could be passed as the first parameter. The benefit of this approach is that the
signature of the member function enforces the arity and types taken by the `__invoke` signature it generates. However
nothing in the `INVOKE` specification requires this behavior.

My solution is to use a `__invoke_enable_if<PM_Type, Tp>`  metafunction to selectively enable the `__invoke` overloads for bullets 1, 2, 3 and 4.  It uses `__member_function_traits` to inspect and extract the return type and class type of the pointer to member. Using `__member_function_traits` to inspect `PM_Type` also allows us to reduce the number of `__invoke` overloads from 32 to 8 and add
varargs support at the same time.

Because `__invoke_enable_if` knows the exact return type of `__invoke` for bullets 1-4 we no longer need to use `decltype(__invoke(...))` to
compute the return type in the `__invoke_return*` traits. This will reduce the problems caused by `#define decltype(X) __typeof__(X)` in C++03.

Tests for this change have already been committed. All tests in `test/std/utilities/function.objects` now pass in C++03, previously there were 20 failures.

Reviewers: K-ballo, howard.hinnant, mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D11553

llvm-svn: 246068

2 files changed, 4 insertions, 0 deletions

diff --git a/libcxx/test/std/utilities/function.objects/refwrap/refwrap.const/type_ctor.fail.cpp b/libcxx/test/std/utilities/function.objects/refwrap/refwrap.const/type_ctor.fail.cpp
index ba46946aae1..a2316063cec 100644
--- a/libcxx/test/std/utilities/function.objects/refwrap/refwrap.const/type_ctor.fail.cpp
+++ b/libcxx/test/std/utilities/function.objects/refwrap/refwrap.const/type_ctor.fail.cpp
@@ -13,6 +13,8 @@
 
 // reference_wrapper(T&&) = delete;
 
+// XFAIL: c++98, c++03
+
 #include <functional>
 #include <cassert>
 
diff --git a/libcxx/test/std/utilities/function.objects/refwrap/refwrap.helpers/ref_1.fail.cpp b/libcxx/test/std/utilities/function.objects/refwrap/refwrap.helpers/ref_1.fail.cpp
index 86a5696f48c..0aad4986a1f 100644
--- a/libcxx/test/std/utilities/function.objects/refwrap/refwrap.helpers/ref_1.fail.cpp
+++ b/libcxx/test/std/utilities/function.objects/refwrap/refwrap.helpers/ref_1.fail.cpp
@@ -15,6 +15,8 @@
 
 // Don't allow binding to a temp
 
+// XFAIL: c++98, c++03
+
 #include <functional>
 
 struct A {};