Mark non-modifying container algorithms as constexpr for C++20.

This change marks Abseil's non-modifying sequence operations including
absl::linear_search and absl::c_linear_search as constexpr when building with
C++20.
PiperOrigin-RevId: 659812405
Change-Id: I8dc2cee873f30531b2eb8fb3da12085505a43a1a

Author: Abseil Team

absl/algorithm/algorithm.h

@@ -53,8 +53,8 @@ using std::rotate;
 // n = (`last` - `first`) comparisons. A linear search over short containers
 // may be faster than a binary search, even when the container is sorted.
 template <typename InputIterator, typename EqualityComparable>
-bool linear_search(InputIterator first, InputIterator last,
-                   const EqualityComparable& value) {
+ABSL_INTERNAL_CONSTEXPR_SINCE_CXX20 bool linear_search(
+    InputIterator first, InputIterator last, const EqualityComparable& value) {
   return std::find(first, last, value) != last;
 }
 

absl/algorithm/algorithm_test.cc

@@ -14,11 +14,9 @@
 
 #include "absl/algorithm/algorithm.h"
 
-#include <algorithm>
-#include <list>
+#include <array>
 #include <vector>
 
-#include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/base/config.h"
 
@@ -47,4 +45,16 @@ TEST_F(LinearSearchTest, linear_searchConst) {
       absl::linear_search(const_container->begin(), const_container->end(), 4));
 }
 
+#if defined(ABSL_INTERNAL_CPLUSPLUS_LANG) && \
+    ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+
+TEST_F(LinearSearchTest, Constexpr) {
+  static constexpr std::array<int, 3> kArray = {1, 2, 3};
+  static_assert(absl::linear_search(kArray.begin(), kArray.end(), 3));
+  static_assert(!absl::linear_search(kArray.begin(), kArray.end(), 4));
+}
+
+#endif  // defined(ABSL_INTERNAL_CPLUSPLUS_LANG) &&
+        //  ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+
 }  // namespace