adding is_non_positive traits

Summary:
template <typename SomeInt>
void foo(SomeInt x) {
// yields an error in clang when SomeInt is unsigned and -Werror is used
if(x <= 0) {
//...
}
}

Test Plan: added unit tests

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D735735

folly/Traits.h

@@ -278,9 +278,9 @@ struct IsOneOf<T, T1, Ts...> {
 };
 
 /*
- * Complementary type traits to check for a negative value.
+ * Complementary type traits to check for a negative/non-positive value.
  *
- * if(x < 0) yields an error in clang for unsigned types when -Werror is used
+ * `if(x < 0)` yields an error in clang for unsigned types when -Werror is used
  */
 
 namespace detail {
@@ -297,11 +297,16 @@ struct is_negative_impl<T, false> {
 
 } // namespace detail {
 
+// same as `x < 0`
 template <typename T>
 constexpr bool is_negative(T x) {
   return folly::detail::is_negative_impl<T, std::is_signed<T>::value>::check(x);
 }
 
+// same as `x <= 0`
+template <typename T>
+constexpr bool is_non_positive(T x) { return !x || folly::is_negative(x); }
+
 } // namespace folly
 
 FOLLY_ASSUME_FBVECTOR_COMPATIBLE_3(std::basic_string);

folly/test/TraitsTest.cpp

 /*
- * Copyright 2012 Facebook, Inc.
+ * Copyright 2013 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -82,6 +82,20 @@ TEST(Traits, bitAndInit) {
   EXPECT_FALSE(IsZeroInitializable<vector<int>>::value);
 }
 
+TEST(Traits, is_negative) {
+  EXPECT_TRUE(folly::is_negative(-1));
+  EXPECT_FALSE(folly::is_negative(0));
+  EXPECT_FALSE(folly::is_negative(1));
+  EXPECT_FALSE(folly::is_negative(0u));
+  EXPECT_FALSE(folly::is_negative(1u));
+
+  EXPECT_TRUE(folly::is_non_positive(-1));
+  EXPECT_TRUE(folly::is_non_positive(0));
+  EXPECT_FALSE(folly::is_non_positive(1));
+  EXPECT_TRUE(folly::is_non_positive(0u));
+  EXPECT_FALSE(folly::is_non_positive(1u));
+}
+
 int main(int argc, char ** argv) {
   testing::InitGoogleTest(&argc, argv);
   google::ParseCommandLineFlags(&argc, &argv, true);