libstdc++: Hide 128-bit int and float types behind handle for basic_format_arg visitation [PR108053]

Implement visit_format_arg and basic_format_arg::visit function,
in terms of  _M_visit_user member functions, that wraps any type
stored inside basic_format_arg, that is not specified in the standard,
into the handle. This affects __int128, unsigned __int128,
PowerPC specific __ieee128 and __ibm128, and _Float128 for architectures
where long double is not 128bits.

The bfloat16, _Float16, _Float32, _Float32, and _Float128 for
128bits long double are not are not addressed, as they
are transformed into a standard floating point types.

For internal purposes __format::__visit_format_arg function is
used, that provides an unmodified access to stored object.

	PR libstdc++/108053

libstdc++-v3/ChangeLog:

	* include/std/format (basic_format_arg::_M_visit_user):
	Helper function for wrapping extension types into handle.
	(visit_format_arg): Call `_M_visit_user` instead of `_M_visit`.
	(basic_format_arg::visit): As above.
	(__format::__visit_format_arg): Provides direct access to
	values stored in basic_format_arg.
	(__format::__int_from_arg): Use __format::__visit_format_arg
	instead of std::visit_format_arg.
	(_Formatting_scanner::_M_format_arg): As above.
	(_Checking_scanner::__do_vformat_to): As above.
	* testsuite/std/format/arguments/args.cc: New tests.
	* testsuite/std/format/string.cc: Test for using __int128
	as width/precision.

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

2 files changed, 81 insertions, 2 deletions

diff --git a/libstdc++-v3/testsuite/std/format/arguments/args.cc b/libstdc++-v3/testsuite/std/format/arguments/args.cc
index 2cea0a1..4c50bc7 100644
--- a/libstdc++-v3/testsuite/std/format/arguments/args.cc
+++ b/libstdc++-v3/testsuite/std/format/arguments/args.cc
@@ -148,9 +148,82 @@ test_member_visit()
 #endif
 }
 
+template<typename T>
+void test_visited_as_handle()
+{
+  T v{};
+  auto store = std::make_format_args(v);
+  std::format_args args = store;
+
+  constexpr auto is_handle = [](auto arg) {
+    return std::is_same_v<decltype(arg), decltype(args.get(0))::handle>;
+  };
+  VERIFY( std::visit_format_arg(is_handle, args.get(0)) );
+#if __cpp_lib_format >= 202306L // C++26 adds std::basic_format_arg::visit
+  VERIFY( args.get(0).visit(is_handle) );
+#endif
+}
+
+template<typename E, typename S>
+void test_visited_as()
+{
+  auto v = static_cast<S>(1.0);
+  auto store = std::make_format_args(v);
+  std::format_args args = store;
+
+  auto is_expected_val = [v](auto arg) {
+    if constexpr (std::is_same_v<decltype(arg), E>)
+      return arg == static_cast<E>(v);
+    return false;
+  };
+  VERIFY( std::visit_format_arg(is_expected_val, args.get(0)) );
+#if __cpp_lib_format >= 202306L // C++26 adds std::basic_format_arg::visit
+  VERIFY( args.get(0).visit(is_expected_val) );
+#endif
+}
+
+template<typename T>
+concept can_format = std::is_default_constructible_v<std::formatter<T, char>>;
+
 int main()
 {
   test_empty();
   test_args();
   test_member_visit();
+
+#ifdef __SIZEOF_INT128__
+  test_visited_as_handle<__int128>();
+  test_visited_as_handle<unsigned __int128>();
+#endif
+// TODO: This should be visited as handle.
+#ifdef __STDCPP_FLOAT16_T__
+  if constexpr (can_format<_Float16>)
+    test_visited_as<float, _Float16>();
+#endif
+#ifdef __STDCPP_BFLOAT16_T__
+  if constexpr (can_format<__gnu_cxx::__bfloat16_t>)
+    test_visited_as<float, __gnu_cxx::__bfloat16_t>();
+#endif
+#ifdef __FLT32_DIG__
+  if constexpr (can_format<_Float32>)
+    test_visited_as<float, _Float32>();
+#endif
+#ifdef __FLT64_DIG__
+  if constexpr (can_format<_Float64>)
+    test_visited_as<double, _Float64>();
+#endif
+#ifdef __FLT128_DIG__
+  if constexpr (can_format<_Float128>)
+# ifdef _GLIBCXX_LDOUBLE_IS_IEEE_BINARY128
+    test_visited_as<long double, _Float128>();
+# else
+    test_visited_as_handle<_Float128>();
+# endif
+#endif
+#ifdef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
+  if constexpr (!std::is_same_v<__ieee128, long double>)
+    test_visited_as_handle<__ieee128>();
+  if constexpr (!std::is_same_v<__ibm128, long double>)
+    test_visited_as_handle<__ibm128>();
+#endif
 }
diff --git a/libstdc++-v3/testsuite/std/format/string.cc b/libstdc++-v3/testsuite/std/format/string.cc
index d5f3050..ee987a1 100644
--- a/libstdc++-v3/testsuite/std/format/string.cc
+++ b/libstdc++-v3/testsuite/std/format/string.cc
@@ -113,11 +113,14 @@ test_format_spec()
   VERIFY( ! is_format_string_for("{:0c}", 'c') );
   VERIFY( ! is_format_string_for("{:0s}", true) );
 
-  // Dynamic width arg must be an integer type.
+  // Dynamic width arg must be a standar integer type.
   VERIFY( ! is_format_string_for("{:{}d}", 1, 1.5) );
   VERIFY( ! is_format_string_for("{:{}d}", 1, true) );
   VERIFY( ! is_format_string_for("{:{}d}", 1, "str") );
   VERIFY( ! is_format_string_for("{:{}d}", 1, nullptr) );
+#ifdef __SIZEOF_INT128__
+  VERIFY( ! is_format_string_for("{:{}d}", 1, static_cast<__int128>(1)) );
+#endif
 
   // Precision only valid for string and floating-point types.
   VERIFY( ! is_format_string_for("{:.3d}", 1) );
@@ -126,11 +129,14 @@ test_format_spec()
   VERIFY( ! is_format_string_for("{:3.3s}", 'c') );
   VERIFY( ! is_format_string_for("{:3.3p}", nullptr) );
 
-  // Dynamic precision arg must be an integer type.
+  // Dynamic precision arg must be a standard integer type.
   VERIFY( ! is_format_string_for("{:.{}f}", 1.0, 1.5) );
   VERIFY( ! is_format_string_for("{:.{}f}", 1.0, true) );
   VERIFY( ! is_format_string_for("{:.{}f}", 1.0, "str") );
   VERIFY( ! is_format_string_for("{:.{}f}", 1.0, nullptr) );
+#ifdef __SIZEOF_INT128__
+  VERIFY( ! is_format_string_for("{:{}f}", 1.0, static_cast<unsigned __int128>(1)) );
+#endif
 
   // Invalid presentation types for integers.
   VERIFY( ! is_format_string_for("{:f}", 1) );