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libstdc++: Fix constification in range_formatter::format [PR109162]

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The _Rg is deduced to lvalue reference for the lvalue arguments,
and in such case __format::__maybe_const_range<_Rg, _CharT> is always _Rg
(adding const to reference does not change behavior).

Now we correctly check if _Range = remove_reference_t<_Rg> is const
formattable range, furthermore as range_formatter<T> can only format
ranges of values of type (possibly const) _Tp, we additional check if the
remove_cvref_t<range_reference_t<const _Range>> is _Tp.

The range_reference_t<R> and range_reference_t<const R> have different
types (modulo remove_cvref_t) for std::vector<bool> (::reference and bool)
or flat_map<T, U> (pair<const T&, U&> and pair<const T&, const U&>).

	PR libstdc++/109162

libstdc++-v3/ChangeLog:

	* include/std/format (range_formatter::format): Format const range,
	only if reference type is not changed.
	* testsuite/std/format/ranges/formatter.cc: New tests.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
This commit is contained in:
Tomasz Kamiński 2025-04-16 13:39:04 +02:00
parent fa99720e9f
commit aef8797522
2 changed files with 30 additions and 3 deletions
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11
libstdc++-v3/include/std/format
View file

@ -5634,9 +5634,14 @@ namespace __format
typename basic_format_context<_Out, _CharT>::iterator
format(_Rg&& __rg, basic_format_context<_Out, _CharT>& __fc) const
{
using __maybe_const_range
= __format::__maybe_const_range<_Rg, _CharT>;
return _M_format<__maybe_const_range>(__rg, __fc);
using _Range = remove_reference_t<_Rg>;
if constexpr (__format::__const_formattable_range<_Range, _CharT>)
{
using _CRef = ranges::range_reference_t<const _Range>;
if constexpr (same_as<remove_cvref_t<_CRef>, _Tp>)
return _M_format<const _Range>(__rg, __fc);
}
return _M_format(__rg, __fc);
}
private:

22
libstdc++-v3/testsuite/std/format/ranges/formatter.cc
View file

@ -1,5 +1,6 @@
// { dg-do run { target c++23 } }
#include <flat_map>
#include <format>
#include <testsuite_hooks.h>
#include <vector>
@ -138,6 +139,26 @@ test_nested()
VERIFY( res == "+<01; 02; 11; 12>+" );
}
struct MyFlatMap : std::flat_map<int, int>
{
using std::flat_map<int, int>::flat_map;
};
template<typename CharT>
struct std::formatter<MyFlatMap, CharT>
// This cannot apply format BitVector const&, because formatted type would
// be std::pair<int const&, int const&>, and formatter for
// pair<int const&, int> cannot format it.
: std::range_formatter<MyFlatMap::reference>
{};
void test_const_ref_type_mismatch()
{
MyFlatMap m{{1, 11}, {2, 22}};
std::string res = std::format("{:m}", m);
VERIFY( res == "{1: 11, 2: 22}" );
}
template<typename T, typename CharT>
using VectorFormatter = std::formatter<std::vector<T>, CharT>;
@ -146,4 +167,5 @@ int main()
test_outputs<std::range_formatter>();
test_outputs<VectorFormatter>();
test_nested();
test_const_ref_type_mismatch();
}