libstdc++: Fix warnings introduced by type-erasing for chrono commits [PR110739]

The r16-1709-g4b3cefed1a08344495fedec4982d85168bd8173f caused `-Woverflow`
in empty_spec.cc file. This warning is not cause by any issue in shipping
code, and results in taking to much shortcut when implementing a test-only
custom representation type Rep, where long was always used to store a value.
In particular common type for Rep and long long int, was de-facto long.
This is addressed by adding Under template parameter, that controls the type
of stored value, and handling it properly in common_type specializations.
No changes to shipping code are necessary.

Secondly, extracting _M_locale_fmt calls in r16-1712-gcaac94, resulted in __ctx
format parameter no longer being used. This patch removes such parameter
entirely, and replace _FormatContext template parameter, with _OutIter parameter
for __out. For consistency type of the __out is decoupled from _FormatContext,
for functions that still need context:
 * to extract locale (_M_A_a, _M_B_b, _M_c, _M_p, _M_r, _M_subsecs)
 * perform formatting for duration/subseconds (_M_Q, _M_T, _M_S, _M_subsecs)

	PR libstdc++/110739

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (__formatter_chrono::_M_format_to):
	Rename _Out to _OutIter for consistency, and update calls
	to specifier functions.
	(__formatter_chrono::_M_wi, __formatter_chrono::_M_C_y_Y)
	(__formatter_chrono::_M_D_x, __formatter_chrono::_M_d_e)
	(__formatter_chrono::_M_F, __formatter_chrono::_M_g_G)
	(__formatter_chrono::_M_H_I, __formatter_chrono::_M_j)
	(__formatter_chrono::_M_m, __formatter_chrono::_M_M)
	(__formatter_chrono::_M_q, __formatter_chrono::_M_R_X)
	(__formatter_chrono::_M_u_w, __formatter_chrono::_M_U_V_W)
	(__formatter_chrono::_M_z, __formatter_chrono::_M_z):
	Remove _FormatContext parameter, and  introduce _OutIter
	for __out type.
	(__formatter_chrono::_M_a_A, __formatter_chrono::_M_B_b)
	(__formatter_chrono::_M_p, __formatter_chrono::_M_Q)
	(__formatter_chrono::_M_r, __formatter_chrono::_M_S)
	(__formatter_chrono::_M_subsecs, __formatter_chrono::_M_T):
	Introduce separate _OutIter template parameter for __out.
	(__formatter_chrono::_M_c, __formatter_chrono::_M_T):
	Likewise, and adjust calls to specifiers functions.
	* testsuite/std/time/format/empty_spec.cc: Make underlying
	type for Rep configurable.

1 files changed, 26 insertions, 17 deletions

diff --git a/libstdc++-v3/testsuite/std/time/format/empty_spec.cc b/libstdc++-v3/testsuite/std/time/format/empty_spec.cc
index 923df3d..ef1b19d 100644
--- a/libstdc++-v3/testsuite/std/time/format/empty_spec.cc
+++ b/libstdc++-v3/testsuite/std/time/format/empty_spec.cc
@@ -77,15 +77,18 @@ test_padding()
   VERIFY( res == WIDEN("==16 is not a valid month==") );
 }
 
-template<typename Ret = void>
+template<typename Ret = void, typename Under = long>
 struct Rep
 {
   using Return
     = std::conditional_t<std::is_void_v<Ret>, Rep, Ret>;
 
-  Rep(long v = 0) : val(v) {}
+  Rep(Under v = 0) : val(v) {}
 
-  operator long() const
+  template<typename ORet, typename OUnder>
+  Rep(Rep<ORet, OUnder> o) : val(o.val) {}
+
+  operator Under() const
   { return val; }
 
   Return
@@ -119,37 +122,43 @@ struct Rep
   operator<<(std::basic_ostream<CharT>& os, const Rep& t)
   { return os << t.val << WIDEN("[via <<]"); }
 
-  long val;
+  Under val;
+};
+
+template<typename Ret, typename Under1, typename Under2>
+struct std::common_type<Rep<Ret, Under1>, Rep<Ret, Under2>>
+{
+  using type = Rep<Ret, std::common_type_t<Under1, Under2>>;
 };
 
-template<typename Ret, typename Other>
+template<typename Ret, typename Under, typename Other>
   requires std::is_integral_v<Other>
-struct std::common_type<Rep<Ret>, Other>
+struct std::common_type<Rep<Ret, Under>, Other>
 {
-  using type = Rep<Ret>;
+  using type = Rep<Ret, std::common_type_t<Under, Other>>;
 };
 
-template<typename Ret, typename Other>
+template<typename Ret, typename Under, typename Other>
   requires std::is_integral_v<Other>
-struct std::common_type<Other, Rep<Ret>>
-  : std::common_type<Rep<Ret>, Other>
+struct std::common_type<Other, Rep<Ret, Under>>
+  : std::common_type<Rep<Ret, Under>, Other>
 { };
 
-template<typename Ret>
-struct std::numeric_limits<Rep<Ret>>
-  : std::numeric_limits<long>
+template<typename Ret, typename Under>
+struct std::numeric_limits<Rep<Ret, Under>>
+  : std::numeric_limits<Under>
 { };
 
-template<typename Ret, typename CharT>
-struct std::formatter<Rep<Ret>, CharT>
-  : std::formatter<long, CharT>
+template<typename Ret, typename Under, typename CharT>
+struct std::formatter<Rep<Ret, Under>, CharT>
+  : std::formatter<Under, CharT>
 {
   template<typename Out>
   typename std::basic_format_context<Out, CharT>::iterator
   format(const Rep<Ret>& t, std::basic_format_context<Out, CharT>& ctx) const
   {
     constexpr std::basic_string_view<CharT> suffix = WIDEN("[via format]");
-    auto out = std::formatter<long, CharT>::format(t.val, ctx);
+    auto out = std::formatter<Under, CharT>::format(t.val, ctx);
     return std::ranges::copy(suffix, out).out;
   }
 };