Fix format_as for non-const begin/end views (#3955)

base::format does not accept rvalues, using the result of format_as
directly means it is passed one. Doesn't matter for ranges with a const
begin and end, but filter_view caches, so it only has mutable begin/end.
Use auto&& to avoid copy if format_as returns const ref

include/fmt/format.h

@@ -4000,7 +4000,9 @@ struct formatter<T, Char, enable_if_t<detail::has_format_as<T>::value>>
   template <typename FormatContext>
   auto format(const T& value, FormatContext& ctx) const -> decltype(ctx.out()) {
     using base = formatter<detail::format_as_t<T>, Char>;
-    return base::format(format_as(value), ctx);
+    // format functions expect lvalue refs, not rvalues
+    auto&& val = format_as(value);
+    return base::format(val, ctx);
   }
 };
 

test/ranges-test.cc

@@ -7,6 +7,7 @@
 
 #include "fmt/ranges.h"
 
+#include <array>
 #include <list>
 #include <map>
 #include <numeric>
@@ -654,18 +655,18 @@ TEST(ranges_test, lvalue_qualified_begin_end) {
 }
 
 #if !defined(__cpp_lib_ranges) || __cpp_lib_ranges <= 202106L
-#  define ENABLE_INPUT_RANGE_JOIN_TEST 0
+#  define ENABLE_STD_VIEWS_TESTS 0
 #elif FMT_CLANG_VERSION
 #  if FMT_CLANG_VERSION > 1500
-#    define ENABLE_INPUT_RANGE_JOIN_TEST 1
+#    define ENABLE_STD_VIEWS_TESTS 1
 #  else
-#    define ENABLE_INPUT_RANGE_JOIN_TEST 0
+#    define ENABLE_STD_VIEWS_TESTS 0
 #  endif
 #else
-#  define ENABLE_INPUT_RANGE_JOIN_TEST 1
+#  define ENABLE_STD_VIEWS_TESTS 1
 #endif
 
-#if ENABLE_INPUT_RANGE_JOIN_TEST
+#if ENABLE_STD_VIEWS_TESTS
 TEST(ranges_test, input_range_join) {
   auto iss = std::istringstream("1 2 3 4 5");
   auto view = std::views::istream<std::string>(iss);
@@ -679,6 +680,42 @@ TEST(ranges_test, input_range_join_overload) {
       "1.2.3.4.5",
       fmt::format("{}", fmt::join(std::views::istream<std::string>(iss), ".")));
 }
+
+namespace views_filter_view_test {
+struct codec_mask {
+  static constexpr auto kCodecs = std::array{0, 1, 2, 3};
+  int except{};
+};
+
+auto format_as(codec_mask mask) {
+  // Careful not to capture param by reference here, it will dangle
+  return codec_mask::kCodecs |
+         std::views::filter([mask](auto c) { return c != mask.except; });
+}
+}  // namespace views_filter_view_test
+
+TEST(ranges_test, format_as_with_ranges_mutable_begin_end) {
+  using namespace views_filter_view_test;
+  {
+    auto make_filter_view = []() {
+      return codec_mask::kCodecs |
+             std::views::filter([](auto c) { return c != 2; });
+    };
+    auto r = make_filter_view();
+    EXPECT_EQ("[0, 1, 3]", fmt::format("{}", r));
+    EXPECT_EQ("[0, 1, 3]", fmt::format("{}", make_filter_view()));
+  }
+
+  {
+    const codec_mask const_mask{2};
+    codec_mask mask{2};
+
+    EXPECT_EQ("[0, 1, 3]", fmt::format("{}", const_mask));
+    EXPECT_EQ("[0, 1, 3]", fmt::format("{}", mask));
+    EXPECT_EQ("[0, 1, 3]", fmt::format("{}", codec_mask{2}));
+  }
+}
+
 #endif
 
 TEST(ranges_test, std_istream_iterator_join) {