Enable fmt::join for uncopyable iterators (#3946)

If iterator not copyable mutate the underlying iterator
Notably std::ranges::basic_istream_view::__iterator
Addresses issue (#3802)

include/fmt/ranges.h

@@ -13,6 +13,7 @@
 #  include <iterator>
 #  include <tuple>
 #  include <type_traits>
+#  include <utility>
 #endif
 
 #include "format.h"
@@ -611,7 +612,7 @@ struct join_view : detail::view {
   basic_string_view<Char> sep;
 
   join_view(It b, Sentinel e, basic_string_view<Char> s)
-      : begin(b), end(e), sep(s) {}
+      : begin(std::move(b)), end(e), sep(s) {}
 };
 
 template <typename It, typename Sentinel, typename Char>
@@ -631,10 +632,25 @@ struct formatter<join_view<It, Sentinel, Char>, Char> {
     return value_formatter_.parse(ctx);
   }
 
-  template <typename FormatContext>
-  auto format(const join_view<It, Sentinel, Char>& value,
+  template <typename FormatContext, typename Iter,
+            FMT_ENABLE_IF(std::is_copy_constructible<Iter>::value)>
+  auto format(const join_view<Iter, Sentinel, Char>& value,
               FormatContext& ctx) const -> decltype(ctx.out()) {
     auto it = value.begin;
+    return do_format(value, ctx, it);
+  }
+
+  template <typename FormatContext, typename Iter,
+            FMT_ENABLE_IF(!std::is_copy_constructible<Iter>::value)>
+  auto format(join_view<Iter, Sentinel, Char>& value, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
+    return do_format(value, ctx, value.begin);
+  }
+
+ private:
+  template <typename FormatContext>
+  auto do_format(const join_view<It, Sentinel, Char>& value, FormatContext& ctx,
+                 It& it) const -> decltype(ctx.out()) {
     auto out = ctx.out();
     if (it != value.end) {
       out = value_formatter_.format(*it, ctx);
@@ -656,7 +672,7 @@ struct formatter<join_view<It, Sentinel, Char>, Char> {
  */
 template <typename It, typename Sentinel>
 auto join(It begin, Sentinel end, string_view sep) -> join_view<It, Sentinel> {
-  return {begin, end, sep};
+  return {std::move(begin), end, sep};
 }
 
 /**

test/ranges-test.cc

@@ -652,3 +652,56 @@ struct lvalue_qualified_begin_end {
 TEST(ranges_test, lvalue_qualified_begin_end) {
   EXPECT_EQ(fmt::format("{}", lvalue_qualified_begin_end{}), "[1, 2, 3, 4, 5]");
 }
+
+#if !defined(__cpp_lib_ranges) || __cpp_lib_ranges <= 202106L
+#  define ENABLE_INPUT_RANGE_JOIN_TEST 0
+#elif FMT_CLANG_VERSION
+#  if FMT_CLANG_VERSION > 1500
+#    define ENABLE_INPUT_RANGE_JOIN_TEST 1
+#  else
+#    define ENABLE_INPUT_RANGE_JOIN_TEST 0
+#  endif
+#else
+#  define ENABLE_INPUT_RANGE_JOIN_TEST 1
+#endif
+
+#if ENABLE_INPUT_RANGE_JOIN_TEST
+TEST(ranges_test, input_range_join) {
+  std::istringstream iss("1 2 3 4 5");
+  auto view = std::views::istream<std::string>(iss);
+  auto joined_view = fmt::join(view.begin(), view.end(), ", ");
+  EXPECT_EQ("1, 2, 3, 4, 5", fmt::format("{}", std::move(joined_view)));
+}
+
+TEST(ranges_test, input_range_join_overload) {
+  std::istringstream iss("1 2 3 4 5");
+  EXPECT_EQ(
+      "1.2.3.4.5",
+      fmt::format("{}", fmt::join(std::views::istream<std::string>(iss), ".")));
+}
+#endif
+
+TEST(ranges_test, std_istream_iterator_join) {
+  std::istringstream iss("1 2 3 4 5");
+  std::istream_iterator<int> first{iss};
+  std::istream_iterator<int> last{};
+  auto joined_view = fmt::join(first, last, ", ");
+  EXPECT_EQ("1, 2, 3, 4, 5", fmt::format("{}", std::move(joined_view)));
+}
+
+TEST(ranges_test, movable_only_istream_iter_join) {
+  // Mirrors c++20 std::ranges::basic_istream_view::iterator
+  struct UncopyableIstreamIter : std::istream_iterator<int> {
+    explicit UncopyableIstreamIter(std::istringstream& iss)
+        : std::istream_iterator<int>{iss} {}
+    UncopyableIstreamIter(const UncopyableIstreamIter&) = delete;
+    UncopyableIstreamIter(UncopyableIstreamIter&&) = default;
+  };
+  static_assert(!std::is_copy_constructible<UncopyableIstreamIter>::value, "");
+
+  std::istringstream iss("1 2 3 4 5");
+  UncopyableIstreamIter first{iss};
+  std::istream_iterator<int> last{};
+  auto joined_view = fmt::join(std::move(first), last, ", ");
+  EXPECT_EQ("1, 2, 3, 4, 5", fmt::format("{}", std::move(joined_view)));
+}