Fix formatting of ranges with begin()&/end()&

C++20 allows ranges to have lvalue-qualified begin() and end() member functions. fmt correctly handles this if begin() and end() are additionally const-qualifed (i.e. begin() const&), but not in the non-const case. For example:

https://godbolt.org/z/YfxaYz5r7

This patch fixes fmt's range detection to handle this case by testing calls to detail::ranges_begin()/end() with an lvalue T&, matching the behaviour in the const case.

include/fmt/ranges.h

@@ -132,8 +132,8 @@ struct has_const_begin_end<
 
 template <typename T>
 struct has_mutable_begin_end<
-    T, void_t<decltype(detail::range_begin(std::declval<T>())),
-              decltype(detail::range_end(std::declval<T>())),
+    T, void_t<decltype(detail::range_begin(std::declval<T&>())),
+              decltype(detail::range_end(std::declval<T&>())),
               // the extra int here is because older versions of MSVC don't
               // SFINAE properly unless there are distinct types
               int>> : std::true_type {};

test/ranges-test.cc

@@ -592,3 +592,14 @@ auto format_as(const tieable& t) -> std::tuple<int, double> {
 TEST(ranges_test, format_as_tie) {
   EXPECT_EQ(fmt::format("{}", tieable()), "(3, 0.42)");
 }
+
+struct lvalue_qualified_begin_end {
+  int arr[5] = {1, 2, 3, 4, 5};
+
+  int const* begin() & { return arr; }
+  int const* end() & { return arr + 5; }
+};
+
+TEST(ranges_test, lvalue_qualified_begin_end) {
+  EXPECT_EQ(fmt::format("{}", lvalue_qualified_begin_end{}), "[1, 2, 3, 4, 5]");
+}