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

See fmtlib/fmt#1784.

Add tests that demonstrate the problem and check obvious variations.

include/fmt/format.h

@@ -3420,15 +3420,15 @@ 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>
+arg_join<detail::iterator_t<Range>, detail::sentinel_t<Range>, char>
-join(const Range& range, string_view sep) {
+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>,
+arg_join<detail::iterator_t<Range>, detail::sentinel_t<Range>,
          wchar_t>
-join(const Range& range, wstring_view sep) {
+join(Range&& range, wstring_view sep) {
   return join(std::begin(range), std::end(range), sep);
 }
 

test/ranges-test.cc

@@ -153,3 +153,39 @@ 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, ",")));
+}