[libc++][format] Fix floating point formatting.

[Mordante]

Summary

Fixes llvm.org/PR58714 reported by @jwakely and a similar issue
reported privately by @vitaut.

[ldionne]

Let's cherry-pick this to release/16.x

libcxx/include/__format/formatter_floating_point.h

if (__first == __result.__last) {
     if (__result.__exponent != __result.__last)
       // In scientific mode if there's a radix point it will always be after
       // the first digit. (This is the position __first points at).
       __result.__radix_point = *__first == '.' ? __first : __result.__last;
     else {
       // In fixed mode the algorithm truncates trailing spaces and possibly the
       // radix point. There's no good guess for the position of the radix point
       // therefore scan the output after the first digit.
-
       __result.__radix_point = _VSTD::find(__first, __result.__last, '.');
     }
   }
 
   // clang-format off
   _LIBCPP_ASSERT((__result.__integral != __result.__last) &&
                  (__result.__radix_point == __result.__last || *__result.__radix_point == '.') &&
                  (__result.__exponent == __result.__last || *__result.__exponent == 'e'),
if (__is_general) {
       // https://en.cppreference.com/w/c/io/fprintf
       // Let P equal the precision if nonzero, 6 if the precision is not
       // specified, or 1 if the precision is 0. Then, if a conversion with
       // style E would have an exponent of X:
       int __p = _VSTD::max(1, (__specs.__has_precision() ? __specs.__precision_ : 6));
       if (__result.__exponent == __result.__last)
         // if P > X >= -4, the conversion is with style f or F and precision P - 1 - X.
         // By including the radix point it calculates P - (1 + X)
-        __p -= __result.__radix_point - __buffer.begin();
+        __p -= __result.__radix_point - __result.__integral;
       else
         // otherwise, the conversion is with style e or E and precision P - 1.
         --__p;
 
       ptrdiff_t __precision = (__result.__exponent - __result.__radix_point) - 1;
       if (__precision < __p)
         __buffer.__add_trailing_zeros(__p - __precision);
     }

libcxx/test/std/utilities/format/format.functions/format_tests.h

void format_test_floating_point_default_precision(TestFunction check) {
 
   check(SV("answer is '-nan'"), SV("answer is '{:.6}'"), nan_neg);
   check(SV("answer is '-nan'"), SV("answer is '{:-.6}'"), nan_neg);
   check(SV("answer is '-nan'"), SV("answer is '{:+.6}'"), nan_neg);
   check(SV("answer is '-nan'"), SV("answer is '{: .6}'"), nan_neg);
 
   // *** alternate form **
   // When precision is zero there's no decimal point except when the alternate form is specified.
+  // Note unlike the g and G option the trailing zeros are still removed.
   check(SV("answer is '0'"), SV("answer is '{:.0}'"), F(0));
   check(SV("answer is '0.'"), SV("answer is '{:#.0}'"), F(0));
 
   check(SV("answer is '0.'"), SV("answer is '{:#.6}'"), F(0));
   check(SV("answer is '2.5'"), SV("answer is '{:#.6}'"), F(2.5));
 
   check(SV("answer is 'inf'"), SV("answer is '{:#.6}'"), std::numeric_limits<F>::infinity());
   check(SV("answer is '-inf'"), SV("answer is '{:#.6}'"), -std::numeric_limits<F>::infinity());
void format_test_floating_point_default_precision(TestFunction check) {
   check(SV("answer is '1.2e+06'"), SV("answer is '{:#.{}}'"), 1200000.0, 4);
   check(SV("answer is '1.2e+06'"), SV("answer is '{:#.{}}'"), 1200000.0, 5);
   check(SV("answer is '1.2e+06'"), SV("answer is '{:#.{}}'"), 1200000.0, 6);
 
   // *** locale-specific form ***
   // See locale-specific_form.pass.cpp
 }
 
+template <class F, class CharT, class TestFunction>
+void format_test_floating_point_PR58714(TestFunction check) {
+  check(SV("+1234"), SV("{:+}"), F(1234.0));
+  check(SV("+1.348p+10"), SV("{:+a}"), F(1234.0));
+  check(SV("+1.234000e+03"), SV("{:+e}"), F(1234.0));
+  check(SV("+1234.000000"), SV("{:+f}"), F(1234.0));
+  check(SV("+1234"), SV("{:+g}"), F(1234.0));
+
+  check(SV("1234."), SV("{:#}"), F(1234.0));
+  check(SV("1.348p+10"), SV("{:#a}"), F(1234.0));
+  check(SV("1.234000e+03"), SV("{:#e}"), F(1234.0));
+  check(SV("1234.000000"), SV("{:#f}"), F(1234.0));
+  check(SV("1234.00"), SV("{:#g}"), F(1234.0));
+
+  check(SV("4.e+30"), SV("{:#}"), F(4.0e+30));
+  check(SV("1.p+102"), SV("{:#a}"), F(0x4.0p+100));
+  check(SV("4.000000e+30"), SV("{:#e}"), F(4.0e+30));
+  check(SV("5070602400912917605986812821504.000000"), SV("{:#f}"), F(0x4.0p+100));
+  check(SV("4.00000e+30"), SV("{:#g}"), F(4.0e+30));
+
+  check(SV("1234."), SV("{:#.6}"), F(1234.0)); // # does not restore zeros
+  check(SV("1.348000p+10"), SV("{:#.6a}"), F(1234.0));
+  check(SV("1.234000e+03"), SV("{:#.6e}"), F(1234.0));
+  check(SV("1234.000000"), SV("{:#.6f}"), F(1234.0));
+  check(SV("1234.00"), SV("{:#.6g}"), F(1234.0));
+
+  check(SV("-1234."), SV("{:#}"), F(-1234.0));
+  check(SV("-1.348p+10"), SV("{:#a}"), F(-1234.0));
+  check(SV("-1.234000e+03"), SV("{:#e}"), F(-1234.0));
+  check(SV("-1234.000000"), SV("{:#f}"), F(-1234.0));
+  check(SV("-1234.00"), SV("{:#g}"), F(-1234.0));
+
+  check(SV("-1234."), SV("{:#.6}"), F(-1234.0)); // # does not restore zeros
+  check(SV("-1.348000p+10"), SV("{:#.6a}"), F(-1234.0));
+  check(SV("-1.234000e+03"), SV("{:#.6e}"), F(-1234.0));
+  check(SV("-1234.000000"), SV("{:#.6f}"), F(-1234.0));
+  check(SV("-1234.00"), SV("{:#.6g}"), F(-1234.0));
+
+  check(SV("+1234."), SV("{:+#}"), F(1234.0));
+  check(SV("+1.348p+10"), SV("{:+#a}"), F(1234.0));
+  check(SV("+1.234000e+03"), SV("{:+#e}"), F(1234.0));
+  check(SV("+1234.000000"), SV("{:+#f}"), F(1234.0));
+  check(SV("+1234.00"), SV("{:+#g}"), F(1234.0));
+
+  check(SV("+1234."), SV("{:+#.6}"), F(1234.0)); // # does not restore zeros
+  check(SV("+1.348000p+10"), SV("{:+#.6a}"), F(1234.0));
+  check(SV("+1.234000e+03"), SV("{:+#.6e}"), F(1234.0));
+  check(SV("+1234.000000"), SV("{:+#.6f}"), F(1234.0));
+  check(SV("+1234.00"), SV("{:+#.6g}"), F(1234.0));
+}
+
 template <class F, class CharT, class TestFunction, class ExceptionTest>
 void format_test_floating_point(TestFunction check, ExceptionTest check_exception) {
   format_test_floating_point_hex_lower_case<F, CharT>(check);
   format_test_floating_point_hex_upper_case<F, CharT>(check);
   format_test_floating_point_hex_lower_case_precision<F, CharT>(check);
   format_test_floating_point_hex_upper_case_precision<F, CharT>(check);
 
   format_test_floating_point_scientific_lower_case<F, CharT>(check);
   format_test_floating_point_scientific_upper_case<F, CharT>(check);
 
   format_test_floating_point_fixed_lower_case<F, CharT>(check);
   format_test_floating_point_fixed_upper_case<F, CharT>(check);
 
   format_test_floating_point_general_lower_case<F, CharT>(check);
   format_test_floating_point_general_upper_case<F, CharT>(check);
 
   format_test_floating_point_default<F, CharT>(check);
   format_test_floating_point_default_precision<F, CharT>(check);
 
+  format_test_floating_point_PR58714<F, CharT>(check);
+
   // *** type ***
   for (const auto& fmt : invalid_types<CharT>("aAeEfFgG"))
     check_exception("The format-spec type has a type not supported for a floating-point argument", fmt, F(1));
 }
 
 template <class CharT, class TestFunction, class ExceptionTest>
 void format_test_floating_point(TestFunction check, ExceptionTest check_exception) {
   format_test_floating_point<float, CharT>(check, check_exception);