Make join() handle non-const-only begin/end ranges (#1786)

See fmtlib/fmt#1784.

Add tests that demonstrate the problem and check obvious variations.

include/fmt/format.h

@@ -3420,15 +3420,14 @@ arg_join<It, Sentinel, wchar_t> join(It begin, Sentinel end, wstring_view sep) {
   \endrst
  */
 template <typename Range>
-arg_join<detail::iterator_t<const Range>, detail::sentinel_t<const Range>, char>
-join(const Range& range, string_view sep) {
+arg_join<detail::iterator_t<Range>, detail::sentinel_t<Range>, char> join(
+    Range&& range, string_view sep) {
   return join(std::begin(range), std::end(range), sep);
 }
 
 template <typename Range>
-arg_join<detail::iterator_t<const Range>, detail::sentinel_t<const Range>,
-         wchar_t>
-join(const Range& range, wstring_view sep) {
+arg_join<detail::iterator_t<Range>, detail::sentinel_t<Range>, wchar_t> join(
+    Range&& range, wstring_view sep) {
   return join(std::begin(range), std::end(range), sep);
 }
 

test/ranges-test.cc

@@ -153,3 +153,37 @@ TEST(RangesTest, JoinSentinel) {
   EXPECT_EQ("{'h', 'e', 'l', 'l', 'o'}", fmt::format("{}", hello));
   EXPECT_EQ("h_e_l_l_o", fmt::format("{}", fmt::join(hello, "_")));
 }
+
+// A range that provides non-const only begin()/end() to test fmt::join handles
+// that
+//
+// Some ranges (eg those produced by range-v3's views::filter()) can cache
+// information during iteration so they only provide non-const begin()/end().
+template <typename T> class non_const_only_range {
+ private:
+  std::vector<T> vec;
+
+ public:
+  using const_iterator = typename ::std::vector<T>::const_iterator;
+
+  template <typename... Args>
+  explicit non_const_only_range(Args&&... args)
+      : vec(::std::forward<Args>(args)...) {}
+
+  const_iterator begin() { return vec.begin(); }
+  const_iterator end() { return vec.end(); }
+};
+
+TEST(RangesTest, JoinRange) {
+  non_const_only_range<int> x(3, 0);
+  EXPECT_EQ("0,0,0", fmt::format("{}", fmt::join(x, ",")));
+  EXPECT_EQ("0,0,0",
+            fmt::format("{}", fmt::join(non_const_only_range<int>(3, 0), ",")));
+
+  std::vector<int> y(3, 0);
+  EXPECT_EQ("0,0,0", fmt::format("{}", fmt::join(y, ",")));
+  EXPECT_EQ("0,0,0", fmt::format("{}", fmt::join(std::vector<int>(3, 0), ",")));
+
+  const std::vector<int> z(3, 0);
+  EXPECT_EQ("0,0,0", fmt::format("{}", fmt::join(z, ",")));
+}