Consider ADL begin() and end() when joining ranges (#3824)

Closes #3813

Signed-off-by: Beat Bolli <dev@drbeat.li>

include/fmt/ranges.h

@@ -578,6 +578,22 @@ auto join(It begin, Sentinel end, string_view sep) -> join_view<It, Sentinel> {
   return {begin, end, sep};
 }
 
+namespace detail {
+// ADL helpers for fmt::join()
+namespace adl {
+using std::begin;
+using std::end;
+
+template <typename Range> auto adlbegin(Range& r) -> decltype(begin(r)) {
+  return begin(r);
+}
+
+template <typename Range> auto adlend(Range& r) -> decltype(end(r)) {
+  return end(r);
+}
+}  // namespace adl
+}  // namespace detail
+
 /**
   \rst
   Returns a view that formats `range` with elements separated by `sep`.
@@ -596,8 +612,9 @@ auto join(It begin, Sentinel end, string_view sep) -> join_view<It, Sentinel> {
  */
 template <typename Range>
 auto join(Range&& range, string_view sep)
-    -> join_view<decltype(std::begin(range)), decltype(std::end(range))> {
-  return join(std::begin(range), std::end(range), sep);
+    -> join_view<decltype(detail::adl::adlbegin(range)),
+                 decltype(detail::adl::adlend(range))> {
+  return join(detail::adl::adlbegin(range), detail::adl::adlend(range), sep);
 }
 
 template <typename Char, typename... T> struct tuple_join_view : detail::view {

test/ranges-test.cc

@@ -455,6 +455,22 @@ TEST(ranges_test, join_range) {
       "0,1,2,3,4");
 #  endif
 }
+
+namespace adl {
+struct vec : std::vector<int> {
+  using std::vector<int>::vector;  // inherit all constructors
+};
+
+// ADL-found begin() and end() skip the first and last element
+auto begin(vec& v) -> typename vec::iterator { return v.begin() + 1; }
+auto end(vec& v) -> typename vec::iterator { return v.end() - 1; }
+}
+
+TEST(ranges_test, format_join_adl_begin_end) {
+  auto v = adl::vec{41, 42, 43, 44};
+  EXPECT_EQ(fmt::format("{}", fmt::join(v, "/")), "42/43");
+}
+
 #endif  // FMT_RANGES_TEST_ENABLE_JOIN
 
 #if defined(__cpp_lib_ranges) && __cpp_lib_ranges >= 202302L