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

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>

2 files changed, 30 insertions, 3 deletions

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index b145597..27253f5 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -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:
diff --git a/libstdc++-v3/testsuite/std/format/ranges/formatter.cc b/libstdc++-v3/testsuite/std/format/ranges/formatter.cc
index a4f5d92..00ce9f6 100644
--- a/libstdc++-v3/testsuite/std/format/ranges/formatter.cc
+++ b/libstdc++-v3/testsuite/std/format/ranges/formatter.cc
@@ -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();
 }