From b6433ce02c8d8a1d4055e06a929e2ab0ce12f882 Mon Sep 17 00:00:00 2001
From: Jeff Walden <jwalden@mit.edu>
Date: Mon, 18 Jun 2012 19:06:33 -0700
Subject: [PATCH] Bug 766347 - Implement a mozilla::EnableIf template struct
 for SFINAE capabilities.  r=luke

---
 mfbt/TypeTraits.h | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/mfbt/TypeTraits.h b/mfbt/TypeTraits.h
index 2d47b2cbf7bae..178ecdf26fc3d 100644
--- a/mfbt/TypeTraits.h
+++ b/mfbt/TypeTraits.h
@@ -49,6 +49,34 @@ struct Conditional<false, A, B>
     typedef B Type;
 };
 
+/*
+ * EnableIf is a struct containing a typedef of T if and only if B is true.
+ *
+ * mozilla::EnableIf<true, int>::Type is int;
+ * mozilla::EnableIf<false, int>::Type is a compile-time error.
+ *
+ * Use this template to implement SFINAE-style (Substitution Failure Is not An
+ * Error) requirements.  For example, you might use it to impose a restriction
+ * on a template parameter:
+ *
+ *   template<typename T>
+ *   class PodVector // vector optimized to store POD (memcpy-able) types
+ *   {
+ *      EnableIf<IsPodType<T>, T>::Type* vector;
+ *      size_t length;
+ *      ...
+ *   };
+ */
+template<bool B, typename T = void>
+struct EnableIf
+{};
+
+template<typename T>
+struct EnableIf<true, T>
+{
+    typedef T Type;
+};
+
 } /* namespace mozilla */
 
 #endif  /* mozilla_TypeTraits_h_ */