libstdc++: Remove dead code in range_formatter::format [PR109162]

Because the _M_format(__rg, __fc) were placed outside of if constexpr,
these method and its children  where instantiated, even if
_M_format<const _Range> could be used.

To simplify the if constexpr chain, we introduce a __simply_formattable_range
(name based on simple-view) exposition only concept, that checks if range is
const and mutable formattable and uses same formatter specialization for
references in each case.

	PR libstdc++/109162

libstdc++-v3/ChangeLog:

	* include/std/format (__format::__simply_formattable_range): Define.
	(range_formatter::format): Do not instantiate _M_format for mutable
	_Rg if const _Rg can be used.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 27253f50ea8dfca819796c513be642c25956f709..2668382c7edba00dabdbbc0991a4cc9a69741554 100644 (file)
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -5252,6 +5252,14 @@ namespace __format
       = ranges::input_range<const _Rg>
          && formattable<ranges::range_reference_t<const _Rg>, _CharT>;
 
+  // _Rg& and const _Rg& are both formattable and use same formatter
+  // specialization for their references.
+  template<typename _Rg, typename _CharT>
+    concept __simply_formattable_range
+      = __const_formattable_range<_Rg, _CharT>
+         && same_as<remove_cvref_t<ranges::range_reference_t<_Rg>>,
+                    remove_cvref_t<ranges::range_reference_t<const _Rg>>>;
+
   template<typename _Rg, typename _CharT>
     using __maybe_const_range
       = __conditional_t<__const_formattable_range<_Rg, _CharT>, const _Rg, _Rg>;
@@ -5635,13 +5643,10 @@ namespace __format
        format(_Rg&& __rg, basic_format_context<_Out, _CharT>& __fc) const
        {
          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);
+         if constexpr (__format::__simply_formattable_range<_Range, _CharT>)
+           return _M_format<const _Range>(__rg, __fc);
+         else
+           return _M_format(__rg, __fc);
        }
 
     private: