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Fix std.h/ranges.h ambiguity for optional in c++26 (fmtlib#4760) #4762

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Fix std.h/ranges.h ambiguity for optional in c++26 (fmtlib#4760) #4762

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13 changes: 13 additions & 0 deletions include/fmt/ranges.h
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Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,13 @@
# include <tuple>
# include <type_traits>
# include <utility>

// Check FMT_CPLUSPLUS to suppress a bogus warning in MSVC.
# if FMT_CPLUSPLUS >= 201703L
# if FMT_HAS_INCLUDE(<optional>)
# include <optional>
# endif
# endif
#endif

#include "format.h"
Expand Down Expand Up @@ -487,6 +494,12 @@ struct range_format_kind
is_range<T, Char>::value, detail::range_format_kind_<T>,
std::integral_constant<range_format, range_format::disabled>> {};

#if defined(__cpp_lib_optional_range_support)
template <typename T, typename Char>
struct range_format_kind<std::optional<T>, Char>
: std::integral_constant<range_format, range_format::disabled> {};
#endif

template <typename R, typename Char>
struct formatter<
R, Char,
Expand Down
12 changes: 12 additions & 0 deletions test/ranges-test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -20,6 +20,10 @@
# include <ranges>
#endif

#if FMT_CPLUSPLUS > 201703L && FMT_HAS_INCLUDE(<optional>)
# include <optional>
#endif

#include "fmt/format.h"
#include "gtest/gtest.h"

Expand Down Expand Up @@ -271,6 +275,14 @@ TEST(ranges_test, disabled_range_formatting_of_path) {
fmt::range_format::disabled);
}

# if defined(__cpp_lib_optional_range_support)
TEST(ranges_test, disabled_range_formatting_of_optional) {
// Range format of optional is disabled to avoid ambiguity with fmt/std.h.
EXPECT_EQ((fmt::range_format_kind<std::optional<int>, char>::value),
fmt::range_format_disabled);
}
#endif

struct vector_string : std::vector<char> {
using base = std::vector<char>;
using base::base;
Expand Down
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