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

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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.
This commit is contained in:
Tristan Brindle 2024-01-10 19:17:28 +00:00 committed by Victor Zverovich
parent 6f5d53ce08
commit 2595bf57b3
2 changed files with 13 additions and 2 deletions
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4
include/fmt/ranges.h
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@ -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 {};

11
test/ranges-test.cc
View File

@ -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]");
}