[STLExtras] Use std::get in less_first,less_second to support more types

[fhahn]

Summary

Update less_first,less_second to use std::get to access the first and
second component. This extends support to any type implementing
std::get, like tuples.

[dblaikie]

Sounds good to me - wouldn't even mind if you wanted to implement a generic abstraction, less<N> and implement less_first/second as aliases of that type.

[fhahn]

In D145489#4175498, @dblaikie wrote:

Sounds good to me - wouldn't even mind if you wanted to implement a generic abstraction, less<N> and implement less_first/second as aliases of that type.

Thanks for taking a look! I am going to land this as is for now, but I can look into generalizing it as follow-up.

llvm/include/llvm/ADT/STLExtras.h

return llvm::map_range(
         return elt.second;
       });
 }
 
 //===----------------------------------------------------------------------===//
 //     Extra additions to <utility>
 //===----------------------------------------------------------------------===//
 
-/// Function object to check whether the first component of a std::pair
-/// compares less than the first component of another std::pair.
+/// Function object to check whether the first component of a container
+/// supported by std::get (like std::pair and std::tuple) compares less than the
+/// first component of another container.
 struct less_first {
   template <typename T> bool operator()(const T &lhs, const T &rhs) const {
-    return std::less<>()(lhs.first, rhs.first);
+    return std::less<>()(std::get<0>(lhs), std::get<0>(rhs));
   }
 };
 
-/// Function object to check whether the second component of a std::pair
-/// compares less than the second component of another std::pair.
+/// Function object to check whether the second component of a container
+/// supported by std::get (like std::pair and std::tuple) compares less than the
+/// second component of another container.
 struct less_second {
   template <typename T> bool operator()(const T &lhs, const T &rhs) const {
-    return std::less<>()(lhs.second, rhs.second);
+    return std::less<>()(std::get<1>(lhs), std::get<1>(rhs));
   }
 };
 
 /// \brief Function object to apply a binary function to the first component of
 /// a std::pair.
 template<typename FuncTy>
 struct on_first {
   FuncTy func;

llvm/unittests/ADT/STLExtrasTest.cpp

 //===- STLExtrasTest.cpp - Unit tests for STL extras ----------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/StringRef.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 
 #include <array>
 #include <climits>
 #include <list>
+#include <tuple>
+#include <utility>
 #include <vector>
 
 using namespace llvm;
 
 using testing::ElementsAre;
 
 namespace {
 
static_assert(addEnumValues(One, C::Two) == 3,
                 "addEnumValues(One, C::Two) failed.");
   // Non-fixed underlying type enum and fixed underlying type enum, with
   // different underlying types
   static_assert(addEnumValues(ULongLongMax, C::Two) ==
                     static_cast<unsigned long long>(ULLONG_MAX) + 2,
                 "addEnumValues(ULongLongMax, C::Two) failed.");
 }
 
+TEST(STLExtrasTest, LessFirst) {
+  {
+    std::pair<int, int> A(0, 1);
+    std::pair<int, int> B(1, 0);
+    EXPECT_TRUE(less_first()(A, B));
+    EXPECT_FALSE(less_first()(B, A));
+  }
+
+  {
+    std::tuple<int, int> A(0, 1);
+    std::tuple<int, int> B(1, 0);
+    EXPECT_TRUE(less_first()(A, B));
+    EXPECT_FALSE(less_first()(B, A));
+  }
+}
+
+TEST(STLExtrasTest, LessSecond) {
+  {
+    std::pair<int, int> A(0, 1);
+    std::pair<int, int> B(1, 0);
+    EXPECT_FALSE(less_second()(A, B));
+    EXPECT_TRUE(less_second()(B, A));
+  }
+
+  {
+    std::tuple<int, int> A(0, 1);
+    std::tuple<int, int> B(1, 0);
+    EXPECT_FALSE(less_second()(A, B));
+    EXPECT_TRUE(less_second()(B, A));
+  }
+}
+
 } // namespace