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Fix format_as for tuple-like types with ranges.h #4945

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Fix format_as for tuple-like types with ranges.h #4945

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63ebcda
Fix format_as for tuple-like types
dajiaohuang Sep 13, 2026
7881fc2
Format format_as tuple trait declaration
dajiaohuang Sep 13, 2026
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8 changes: 7 additions & 1 deletion include/fmt/ranges.h
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Original file line number Diff line number Diff line change
Expand Up @@ -164,7 +164,13 @@ using make_index_sequence = make_integer_sequence<size_t, N>;
template <typename T>
using tuple_index_sequence = make_index_sequence<std::tuple_size<T>::value>;

template <typename T, typename C, bool = is_tuple_like_<T>::value>
template <typename T, typename = void>
struct has_format_as_ : std::false_type {};
template <typename T>
struct has_format_as_<T, void_t<format_as_result<T>>> : std::true_type {};

template <typename T, typename C,
bool = is_tuple_like_<T>::value && !has_format_as_<T>::value>
class is_tuple_formattable_ {
public:
static constexpr bool value = false;
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17 changes: 17 additions & 0 deletions test/ranges-test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -721,6 +721,23 @@ TEST(ranges_test, format_as_tie) {
EXPECT_EQ(fmt::format("{}", tieable()), "(3, 0.42)");
}

struct format_as_tuple_like {};

auto format_as(const format_as_tuple_like&) -> std::string { return "Hello"; }

namespace std {
template <> struct tuple_size<format_as_tuple_like>
: integral_constant<size_t, 1> {};
template <> struct tuple_element<0, format_as_tuple_like> {
using type = format_as_tuple_like;
};
} // namespace std

TEST(ranges_test, format_as_tuple_like) {
EXPECT_TRUE((fmt::is_formattable<format_as_tuple_like>::value));
EXPECT_EQ(fmt::format("{}", format_as_tuple_like{}), "Hello");
}

struct lvalue_qualified_begin_end {
int arr[5] = {1, 2, 3, 4, 5};

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