Fix implementation of ::abs and std::abs LWG 2192.

Summary:
All overloads of `::abs` and `std::abs` must be present in both `<cmath>` and `<cstdlib>`. This is problematic to implement because C defines `fabs` in `math.h` and `labs` in `stdlib.h`. This introduces a circular dependency between the two headers. 

This patch implements that requirement by moving `abs` into `math.h` and making `stdlib.h` include `math.h`. In order to get the underlying C declarations from the "real" `stdlib.h` inside our `math.h` we need some trickery. Specifically we need to make `stdlib.h` include next itself.

Suggestions for a cleaner implementation are welcome.

Reviewers: mclow.lists, ldionne

Reviewed By: ldionne

Subscribers: krytarowski, fedor.sergeev, dexonsmith, jdoerfert, jsji, libcxx-commits

Differential Revision: https://reviews.llvm.org/D60097

llvm-svn: 359020

1 files changed, 46 insertions, 7 deletions

diff --git a/libcxx/test/std/numerics/c.math/cmath.pass.cpp b/libcxx/test/std/numerics/c.math/cmath.pass.cpp
index 3f9a5f557cc..fc953954e5c 100644
--- a/libcxx/test/std/numerics/c.math/cmath.pass.cpp
+++ b/libcxx/test/std/numerics/c.math/cmath.pass.cpp
@@ -100,15 +100,54 @@ Ambiguous scalbn(Ambiguous, Ambiguous){ return Ambiguous(); }
 Ambiguous tgamma(Ambiguous){ return Ambiguous(); }
 Ambiguous trunc(Ambiguous){ return Ambiguous(); }
 
-void test_abs()
-{
-    static_assert((std::is_same<decltype(std::abs((float)0)), float>::value), "");
-    static_assert((std::is_same<decltype(std::abs((double)0)), double>::value), "");
-    static_assert((std::is_same<decltype(std::abs((long double)0)), long double>::value), "");
-    static_assert((std::is_same<decltype(abs(Ambiguous())), Ambiguous>::value), "");
-    assert(std::abs(-1.) == 1);
+template <class T, class = decltype(std::abs(std::declval<T>()))>
+std::true_type has_abs_imp(int);
+template <class T>
+std::false_type has_abs_imp(...);
+
+template <class T>
+struct has_abs : decltype(has_abs_imp<T>(0)) {};
+
+void test_abs() {
+#ifdef __clang__
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wabsolute-value"
+#endif
+  static_assert((std::is_same<decltype(std::abs((float)0)), float>::value), "");
+  static_assert((std::is_same<decltype(std::abs((double)0)), double>::value), "");
+  static_assert(
+      (std::is_same<decltype(std::abs((long double)0)), long double>::value), "");
+  static_assert((std::is_same<decltype(std::abs((int)0)), int>::value), "");
+  static_assert((std::is_same<decltype(std::abs((long)0)), long>::value), "");
+  static_assert((std::is_same<decltype(std::abs((long long)0)), long long>::value),
+                "");
+  static_assert((std::is_same<decltype(std::abs((unsigned char)0)), int>::value),
+                "");
+  static_assert((std::is_same<decltype(std::abs((unsigned short)0)), int>::value),
+                "");
+  static_assert((std::is_same<decltype(std::abs((signed char)0)), int>::value),
+                "");
+  static_assert((std::is_same<decltype(std::abs((short)0)), int>::value),
+                "");
+  static_assert((std::is_same<decltype(std::abs((unsigned char)0)), int>::value),
+                "");
+  static_assert((std::is_same<decltype(std::abs((char)0)), int>::value),
+                "");
+  static_assert((std::is_same<decltype(abs(Ambiguous())), Ambiguous>::value), "");
+
+  static_assert(!has_abs<unsigned>::value, "");
+  static_assert(!has_abs<unsigned long>::value, "");
+  static_assert(!has_abs<unsigned long long>::value, "");
+  static_assert(!has_abs<size_t>::value, "");
+
+#ifdef __clang__
+#pragma clang diagnostic pop
+#endif
+
+  assert(std::abs(-1.) == 1);
 }
 
+
 void test_acos()
 {
     static_assert((std::is_same<decltype(std::acos((float)0)), float>::value), "");