fix universal: make enumerate consistent with respect to const

Author: ddvamp

universal/include/userver/utils/enumerate.hpp

@@ -49,26 +49,22 @@ struct IteratorWrapper {
     }
 };
 
-template <typename Range>
-using IteratorTypeOf = decltype(std::begin(std::declval<Range&>()));
-
-template <typename Range>
-using SentinelTypeOf = decltype(std::end(std::declval<Range&>()));
-
 template <typename Container>
 struct ContainerWrapper {
-    constexpr IteratorWrapper<IteratorTypeOf<Container>> begin() {
-        return {.iterator = std::begin(container), .pos = 0};
+    constexpr auto begin() {
+        return IteratorWrapper{.iterator = std::begin(container), .pos = 0};
     }
 
-    constexpr IteratorWrapper<SentinelTypeOf<Container>> end() { return {.iterator = std::end(container), .pos = 0}; }
+    constexpr auto end() {
+        return IteratorWrapper{.iterator = std::end(container), .pos = 0};
+    }
 
-    constexpr IteratorWrapper<IteratorTypeOf<const Container>> begin() const {
-        return {.iterator = std::begin(container), .pos = 0};
+    constexpr auto begin() const {
+        return IteratorWrapper{.iterator = std::begin(std::as_const(container)), .pos = 0};
     }
 
-    constexpr IteratorWrapper<SentinelTypeOf<const Container>> end() const {
-        return {.iterator = std::end(container), .pos = 0};
+    constexpr auto end() const {
+        return IteratorWrapper{.iterator = std::end(std::as_const(container)), .pos = 0};
     }
 
     Container container;
@@ -81,9 +77,9 @@ namespace utils {
 /// @brief Implementation of python-style enumerate function for range-for loops
 /// @param iterable: Container to iterate
 /// @returns ContainerWrapper, which iterator after dereference returns pair
-/// of index and (!!!)non-const reference to element(it seems impossible to make
-/// this reference const). It can be used in "range based for loop" with
-/// "structured binding" like this
+/// of index and reference to element. The reference is const-qualified if either
+/// the wrapper itself or the underlying container is const; otherwise, it is non-const.
+/// It can be used in "range based for loop" with "structured binding" like this
 /// @code
 /// for (auto [pos, elem] : enumerate(someContainer)) {...}
 /// @endcode

universal/src/utils/enumerate_test.cpp

@@ -1,4 +1,6 @@
 #include <array>
+#include <type_traits>
+#include <utility>
 
 #include <gmock/gmock.h>
 #include <gtest/gtest.h>
@@ -9,6 +11,31 @@ USERVER_NAMESPACE_BEGIN
 
 namespace {
 
+template <typename Container>
+using EnumerateImpl = decltype(utils::enumerate(std::declval<Container>()));
+
+template <typename Container, bool ConstWrapper>
+using Enumerate = std::conditional_t<ConstWrapper, const EnumerateImpl<Container>, EnumerateImpl<Container>>;
+
+template <typename Container, bool ConstWrapper>
+using EnumerateElem = decltype(*std::begin(std::declval<Enumerate<Container, ConstWrapper>&>()))::second_type;
+
+static_assert(std::is_same_v<int&, EnumerateElem<std::vector<int>, false>>);
+static_assert(std::is_same_v<int&, EnumerateElem<std::vector<int>&, false>>);
+static_assert(std::is_same_v<int&, EnumerateElem<std::vector<int>&&, false>>);
+
+static_assert(std::is_same_v<const int&, EnumerateElem<const std::vector<int>, false>>);
+static_assert(std::is_same_v<const int&, EnumerateElem<const std::vector<int>&, false>>);
+static_assert(std::is_same_v<const int&, EnumerateElem<const std::vector<int>&&, false>>);
+
+static_assert(std::is_same_v<const int&, EnumerateElem<std::vector<int>, true>>);
+static_assert(std::is_same_v<const int&, EnumerateElem<std::vector<int>&, true>>);
+static_assert(std::is_same_v<const int&, EnumerateElem<std::vector<int>&&, true>>);
+
+static_assert(std::is_same_v<const int&, EnumerateElem<const std::vector<int>, true>>);
+static_assert(std::is_same_v<const int&, EnumerateElem<const std::vector<int>&, true>>);
+static_assert(std::is_same_v<const int&, EnumerateElem<const std::vector<int>&&, true>>);
+
 constexpr int ConstexprTest(std::array<int, 2> data) {
     int result = 0;
     for (auto [pos, elem] : utils::enumerate(data)) {