[libc++] LWG2381: Inconsistency in parsing floating point numbers (#77948)

This PR implements [LWG2381](https://cplusplus.github.io/LWG/issue2381)
by rejecting `'i'`, `'I'`, `'n'`, `'N'` in FP parsing, as inf and NaN
are intendedly rejected by that LWG issue.

The source character array used for parsing is
`"0123456789abcdefABCDEFxX+-pPiInN"`, whose first 26 or 28 characters
are used for parsing integers or floating-point values respectively.
Previously, libc++ used 32 characters, including `'i'`, `'I'`, `'n'`,
`'N'`, for FP parsing, which was inconsistent with LWG2381. This PR also
replaces magic numbers 26 and 28 (formerly 32) with named constants.

Drive-by change: when the first character (possibly after the leading
`'+'` or `'-'`) is not a decimal digit but an acceptable character
(e.g., `'p'` or `'e'`), the character is not accumulated now (per Stage
2 in [facet.num.get.virtuals]/3).

#65168 may be rendered invalid, see
https://github.com/llvm/llvm-project/pull/65168#issuecomment-1868533342.

Apple back-deployment targets remain broken, likely due to dylib. XFAIL
is marked in related tests.

---------

Co-authored-by: Mark de Wever <koraq@xs4all.nl>

5 files changed, 467 insertions, 71 deletions

diff --git a/libcxx/docs/Status/Cxx23Issues.csv b/libcxx/docs/Status/Cxx23Issues.csv
index d421fee..cc601b3 100644
--- a/libcxx/docs/Status/Cxx23Issues.csv
+++ b/libcxx/docs/Status/Cxx23Issues.csv
@@ -98,7 +98,7 @@
 `3555 <https://wg21.link/LWG3555>`__,"``{transform,elements}_view::iterator::iterator_concept`` should consider const-qualification of the underlying range","June 2021","","","|ranges|"
 "","","","","",""
 `2191 <https://wg21.link/LWG2191>`__,"Incorrect specification of ``match_results(match_results&&)``","October 2021","|Nothing To Do|",""
-`2381 <https://wg21.link/LWG2381>`__,"Inconsistency in parsing floating point numbers","October 2021","",""
+`2381 <https://wg21.link/LWG2381>`__,"Inconsistency in parsing floating point numbers","October 2021","|Complete|","19.0"
 `2762 <https://wg21.link/LWG2762>`__,"``unique_ptr operator*()`` should be ``noexcept``","October 2021","",""
 `3121 <https://wg21.link/LWG3121>`__,"``tuple`` constructor constraints for ``UTypes&&...`` overloads","October 2021","",""
 `3123 <https://wg21.link/LWG3123>`__,"``duration`` constructor from representation shouldn't be effectively non-throwing","October 2021","","","|chrono|"
diff --git a/libcxx/include/locale b/libcxx/include/locale
index 748b276..041d7bc 100644
--- a/libcxx/include/locale
+++ b/libcxx/include/locale
@@ -368,7 +368,11 @@ struct _LIBCPP_EXPORTED_FROM_ABI __num_get_base {
   static const int __num_get_buf_sz = 40;
 
   static int __get_base(ios_base&);
-  static const char __src[33];
+  static const char __src[33]; // "0123456789abcdefABCDEFxX+-pPiInN"
+  // count of leading characters in __src used for parsing integers ("012..X+-")
+  static const size_t __int_chr_cnt = 26;
+  // count of leading characters in __src used for parsing floating-point values ("012..-pP")
+  static const size_t __fp_chr_cnt = 28;
 };
 
 _LIBCPP_EXPORTED_FROM_ABI void
@@ -431,7 +435,7 @@ private:
   template <typename _Tp>
   const _Tp* __do_widen_p(ios_base& __iob, _Tp* __atoms) const {
     locale __loc = __iob.getloc();
-    use_facet<ctype<_Tp> >(__loc).widen(__src, __src + 26, __atoms);
+    use_facet<ctype<_Tp> >(__loc).widen(__src, __src + __int_chr_cnt, __atoms);
     return __atoms;
   }
 
@@ -447,7 +451,7 @@ private:
 template <class _CharT>
 string __num_get<_CharT>::__stage2_int_prep(ios_base& __iob, _CharT* __atoms, _CharT& __thousands_sep) {
   locale __loc = __iob.getloc();
-  std::use_facet<ctype<_CharT> >(__loc).widen(__src, __src + 26, __atoms);
+  std::use_facet<ctype<_CharT> >(__loc).widen(__src, __src + __int_chr_cnt, __atoms);
   const numpunct<_CharT>& __np = std::use_facet<numpunct<_CharT> >(__loc);
   __thousands_sep              = __np.thousands_sep();
   return __np.grouping();
@@ -458,7 +462,7 @@ template <class _CharT>
 string __num_get<_CharT>::__stage2_float_prep(
     ios_base& __iob, _CharT* __atoms, _CharT& __decimal_point, _CharT& __thousands_sep) {
   locale __loc = __iob.getloc();
-  std::use_facet<ctype<_CharT> >(__loc).widen(__src, __src + 32, __atoms);
+  std::use_facet<ctype<_CharT> >(__loc).widen(__src, __src + __fp_chr_cnt, __atoms);
   const numpunct<_CharT>& __np = std::use_facet<numpunct<_CharT> >(__loc);
   __decimal_point              = __np.decimal_point();
   __thousands_sep              = __np.thousands_sep();
@@ -490,7 +494,7 @@ __num_get<_CharT>::__stage2_int_loop(_CharT __ct, int __base, char* __a, char*&
     }
     return 0;
   }
-  ptrdiff_t __f = std::find(__atoms, __atoms + 26, __ct) - __atoms;
+  ptrdiff_t __f = std::find(__atoms, __atoms + __int_chr_cnt, __ct) - __atoms;
   if (__f >= 24)
     return -1;
   switch (__base) {
@@ -546,8 +550,8 @@ int __num_get<_CharT>::__stage2_float_loop(
     }
     return 0;
   }
-  ptrdiff_t __f = std::find(__atoms, __atoms + 32, __ct) - __atoms;
-  if (__f >= 32)
+  ptrdiff_t __f = std::find(__atoms, __atoms + __num_get_base::__fp_chr_cnt, __ct) - __atoms;
+  if (__f >= static_cast<ptrdiff_t>(__num_get_base::__fp_chr_cnt))
     return -1;
   char __x = __src[__f];
   if (__x == '-' || __x == '+') {
@@ -846,7 +850,7 @@ _InputIterator num_get<_CharT, _InputIterator>::__do_get_signed(
   int __base = this->__get_base(__iob);
   // Stage 2
   char_type __thousands_sep;
-  const int __atoms_size = 26;
+  const int __atoms_size = __num_get_base::__int_chr_cnt;
 #ifdef _LIBCPP_ABI_OPTIMIZED_LOCALE_NUM_GET
   char_type __atoms1[__atoms_size];
   const char_type* __atoms = this->__do_widen(__iob, __atoms1);
@@ -895,7 +899,7 @@ _InputIterator num_get<_CharT, _InputIterator>::__do_get_unsigned(
   int __base = this->__get_base(__iob);
   // Stage 2
   char_type __thousands_sep;
-  const int __atoms_size = 26;
+  const int __atoms_size = __num_get_base::__int_chr_cnt;
 #ifdef _LIBCPP_ABI_OPTIMIZED_LOCALE_NUM_GET
   char_type __atoms1[__atoms_size];
   const char_type* __atoms = this->__do_widen(__iob, __atoms1);
@@ -942,7 +946,7 @@ _InputIterator num_get<_CharT, _InputIterator>::__do_get_floating_point(
     iter_type __b, iter_type __e, ios_base& __iob, ios_base::iostate& __err, _Fp& __v) const {
   // Stage 1, nothing to do
   // Stage 2
-  char_type __atoms[32];
+  char_type __atoms[__num_get_base::__fp_chr_cnt];
   char_type __decimal_point;
   char_type __thousands_sep;
   string __grouping = this->__stage2_float_prep(__iob, __atoms, __decimal_point, __thousands_sep);
@@ -951,10 +955,11 @@ _InputIterator num_get<_CharT, _InputIterator>::__do_get_floating_point(
   char* __a     = &__buf[0];
   char* __a_end = __a;
   unsigned __g[__num_get_base::__num_get_buf_sz];
-  unsigned* __g_end = __g;
-  unsigned __dc     = 0;
-  bool __in_units   = true;
-  char __exp        = 'E';
+  unsigned* __g_end        = __g;
+  unsigned __dc            = 0;
+  bool __in_units          = true;
+  char __exp               = 'E';
+  bool __is_leading_parsed = false;
   for (; __b != __e; ++__b) {
     if (__a_end == __a + __buf.size()) {
       size_t __tmp = __buf.size();
@@ -977,6 +982,21 @@ _InputIterator num_get<_CharT, _InputIterator>::__do_get_floating_point(
             __dc,
             __atoms))
       break;
+
+    // the leading character excluding the sign must be a decimal digit
+    if (!__is_leading_parsed) {
+      if (__a_end - __a >= 1 && __a[0] != '-' && __a[0] != '+') {
+        if ('0' <= __a[0] && __a[0] <= '9')
+          __is_leading_parsed = true;
+        else
+          break;
+      } else if (__a_end - __a >= 2 && (__a[0] == '-' || __a[0] == '+')) {
+        if ('0' <= __a[1] && __a[1] <= '9')
+          __is_leading_parsed = true;
+        else
+          break;
+      }
+    }
   }
   if (__grouping.size() != 0 && __in_units && __g_end - __g < __num_get_base::__num_get_buf_sz)
     *__g_end++ = __dc;
@@ -996,10 +1016,11 @@ _InputIterator num_get<_CharT, _InputIterator>::do_get(
   // Stage 1
   int __base = 16;
   // Stage 2
-  char_type __atoms[26];
+  char_type __atoms[__num_get_base::__int_chr_cnt];
   char_type __thousands_sep = char_type();
   string __grouping;
-  std::use_facet<ctype<_CharT> >(__iob.getloc()).widen(__num_get_base::__src, __num_get_base::__src + 26, __atoms);
+  std::use_facet<ctype<_CharT> >(__iob.getloc())
+      .widen(__num_get_base::__src, __num_get_base::__src + __num_get_base::__int_chr_cnt, __atoms);
   string __buf;
   __buf.resize(__buf.capacity());
   char* __a     = &__buf[0];
diff --git a/libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_double.pass.cpp b/libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_double.pass.cpp
index c802ab7..fbd1c7c 100644
--- a/libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_double.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_double.pass.cpp
@@ -6,6 +6,11 @@
 //
 //===----------------------------------------------------------------------===//
 
+// The fix for LWG2381 (https://github.com/llvm/llvm-project/pull/77948) changed
+// behavior of FP parsing, while Apple back-deployment targets remain broken due
+// to the dylib.
+// UNSUPPORTED: using-built-library-before-llvm-19
+
 // <locale>
 
 // class num_get<charT, InputIterator>
@@ -116,9 +121,9 @@ int main(int, char**)
             f.get(cpp17_input_iterator<const char*>(str),
                   cpp17_input_iterator<const char*>(str+sizeof(str)),
                   ios, err, v);
-        assert(base(iter) == str+sizeof(str)-1);
-        assert(err == ios.goodbit);
-        assert(v == INFINITY);
+        assert(base(iter) == str);
+        assert(err == ios.failbit);
+        assert(v == 0.0);
     }
     {
         const char str[] = "INF";
@@ -128,9 +133,9 @@ int main(int, char**)
             f.get(cpp17_input_iterator<const char*>(str),
                   cpp17_input_iterator<const char*>(str+sizeof(str)),
                   ios, err, v);
-        assert(base(iter) == str+sizeof(str)-1);
-        assert(err == ios.goodbit);
-        assert(v == INFINITY);
+        assert(base(iter) == str);
+        assert(err == ios.failbit);
+        assert(v == 0.0);
     }
     {
         const char str[] = "-inf";
@@ -140,9 +145,9 @@ int main(int, char**)
             f.get(cpp17_input_iterator<const char*>(str),
                   cpp17_input_iterator<const char*>(str+sizeof(str)),
                   ios, err, v);
-        assert(base(iter) == str+sizeof(str)-1);
-        assert(err == ios.goodbit);
-        assert(v == -INFINITY);
+        assert(base(iter) == str + 1);
+        assert(err == ios.failbit);
+        assert(v == 0.0);
     }
     {
         const char str[] = "-INF";
@@ -152,9 +157,9 @@ int main(int, char**)
             f.get(cpp17_input_iterator<const char*>(str),
                   cpp17_input_iterator<const char*>(str+sizeof(str)),
                   ios, err, v);
-        assert(base(iter) == str+sizeof(str)-1);
-        assert(err == ios.goodbit);
-        assert(v == -INFINITY);
+        assert(base(iter) == str + 1);
+        assert(err == ios.failbit);
+        assert(v == 0.0);
     }
     {
         const char str[] = "nan";
@@ -164,9 +169,9 @@ int main(int, char**)
             f.get(cpp17_input_iterator<const char*>(str),
                   cpp17_input_iterator<const char*>(str+sizeof(str)),
                   ios, err, v);
-        assert(base(iter) == str+sizeof(str)-1);
-        assert(err == ios.goodbit);
-        assert(std::isnan(v));
+        assert(base(iter) == str);
+        assert(err == ios.failbit);
+        assert(v == 0.0);
     }
     {
         const char str[] = "NAN";
@@ -176,9 +181,129 @@ int main(int, char**)
             f.get(cpp17_input_iterator<const char*>(str),
                   cpp17_input_iterator<const char*>(str+sizeof(str)),
                   ios, err, v);
-        assert(base(iter) == str+sizeof(str)-1);
-        assert(err == ios.goodbit);
-        assert(std::isnan(v));
+        assert(base(iter) == str);
+        assert(err == ios.failbit);
+        assert(v == 0.0);
+    }
+    {
+      const char str[] = "p00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str);
+      assert(err == ios.failbit);
+      assert(v == 0.0);
+    }
+    {
+      const char str[] = "P00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str);
+      assert(err == ios.failbit);
+      assert(v == 0.0);
+    }
+    {
+      const char str[] = "+p00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0);
+    }
+    {
+      const char str[] = "+P00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0);
+    }
+    {
+      const char str[] = "-p00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0);
+    }
+    {
+      const char str[] = "-P00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0);
+    }
+    {
+      const char str[] = "e00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str);
+      assert(err == ios.failbit);
+      assert(v == 0.0);
+    }
+    {
+      const char str[] = "E00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str);
+      assert(err == ios.failbit);
+      assert(v == 0.0);
+    }
+    {
+      const char str[] = "+e00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0);
+    }
+    {
+      const char str[] = "+E00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0);
+    }
+    {
+      const char str[] = "-e00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0);
+    }
+    {
+      const char str[] = "-E00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0);
     }
     {
         v = -1;
diff --git a/libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_float.pass.cpp b/libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_float.pass.cpp
index 79c8480..b5ac7d8 100644
--- a/libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_float.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_float.pass.cpp
@@ -6,6 +6,11 @@
 //
 //===----------------------------------------------------------------------===//
 
+// The fix for LWG2381 (https://github.com/llvm/llvm-project/pull/77948) changed
+// behavior of FP parsing, while Apple back-deployment targets remain broken due
+// to the dylib.
+// UNSUPPORTED: using-built-library-before-llvm-19
+
 // <locale>
 
 // class num_get<charT, InputIterator>
@@ -105,9 +110,9 @@ int main(int, char**)
             f.get(cpp17_input_iterator<const char*>(str),
                   cpp17_input_iterator<const char*>(str+sizeof(str)),
                   ios, err, v);
-        assert(base(iter) == str+sizeof(str)-1);
-        assert(err == ios.goodbit);
-        assert(v == INFINITY);
+        assert(base(iter) == str);
+        assert(err == ios.failbit);
+        assert(v == 0.0f);
     }
     {
         const char str[] = "INF";
@@ -117,9 +122,9 @@ int main(int, char**)
             f.get(cpp17_input_iterator<const char*>(str),
                   cpp17_input_iterator<const char*>(str+sizeof(str)),
                   ios, err, v);
-        assert(base(iter) == str+sizeof(str)-1);
-        assert(err == ios.goodbit);
-        assert(v == INFINITY);
+        assert(base(iter) == str);
+        assert(err == ios.failbit);
+        assert(v == 0.0f);
     }
     {
         const char str[] = "-inf";
@@ -129,9 +134,9 @@ int main(int, char**)
             f.get(cpp17_input_iterator<const char*>(str),
                   cpp17_input_iterator<const char*>(str+sizeof(str)),
                   ios, err, v);
-        assert(base(iter) == str+sizeof(str)-1);
-        assert(err == ios.goodbit);
-        assert(v == -INFINITY);
+        assert(base(iter) == str + 1);
+        assert(err == ios.failbit);
+        assert(v == 0.0f);
     }
     {
         const char str[] = "-INF";
@@ -141,9 +146,9 @@ int main(int, char**)
             f.get(cpp17_input_iterator<const char*>(str),
                   cpp17_input_iterator<const char*>(str+sizeof(str)),
                   ios, err, v);
-        assert(base(iter) == str+sizeof(str)-1);
-        assert(err == ios.goodbit);
-        assert(v == -INFINITY);
+        assert(base(iter) == str + 1);
+        assert(err == ios.failbit);
+        assert(v == 0.0f);
     }
     {
         const char str[] = "nan";
@@ -153,9 +158,9 @@ int main(int, char**)
             f.get(cpp17_input_iterator<const char*>(str),
                   cpp17_input_iterator<const char*>(str+sizeof(str)),
                   ios, err, v);
-        assert(base(iter) == str+sizeof(str)-1);
-        assert(err == ios.goodbit);
-        assert(std::isnan(v));
+        assert(base(iter) == str);
+        assert(err == ios.failbit);
+        assert(v == 0.0f);
     }
     {
         const char str[] = "NAN";
@@ -165,9 +170,129 @@ int main(int, char**)
             f.get(cpp17_input_iterator<const char*>(str),
                   cpp17_input_iterator<const char*>(str+sizeof(str)),
                   ios, err, v);
-        assert(base(iter) == str+sizeof(str)-1);
-        assert(err == ios.goodbit);
-        assert(std::isnan(v));
+        assert(base(iter) == str);
+        assert(err == ios.failbit);
+        assert(v == 0.0f);
+    }
+    {
+      const char str[] = "p00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str);
+      assert(err == ios.failbit);
+      assert(v == 0.0f);
+    }
+    {
+      const char str[] = "P00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str);
+      assert(err == ios.failbit);
+      assert(v == 0.0f);
+    }
+    {
+      const char str[] = "+p00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0f);
+    }
+    {
+      const char str[] = "+P00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0f);
+    }
+    {
+      const char str[] = "-p00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0f);
+    }
+    {
+      const char str[] = "-P00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0f);
+    }
+    {
+      const char str[] = "e00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str);
+      assert(err == ios.failbit);
+      assert(v == 0.0f);
+    }
+    {
+      const char str[] = "E00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str);
+      assert(err == ios.failbit);
+      assert(v == 0.0f);
+    }
+    {
+      const char str[] = "+e00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0f);
+    }
+    {
+      const char str[] = "+E00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0f);
+    }
+    {
+      const char str[] = "-e00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0f);
+    }
+    {
+      const char str[] = "-E00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0f);
     }
     {
         v = -1;
diff --git a/libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_long_double.pass.cpp b/libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_long_double.pass.cpp
index e2b2aeaf..9617899 100644
--- a/libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_long_double.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_long_double.pass.cpp
@@ -6,6 +6,11 @@
 //
 //===----------------------------------------------------------------------===//
 
+// The fix for LWG2381 (https://github.com/llvm/llvm-project/pull/77948) changed
+// behavior of FP parsing, while Apple back-deployment targets remain broken due
+// to the dylib.
+// UNSUPPORTED: using-built-library-before-llvm-19
+
 // <locale>
 
 // class num_get<charT, InputIterator>
@@ -105,9 +110,9 @@ int main(int, char**)
             f.get(cpp17_input_iterator<const char*>(str),
                   cpp17_input_iterator<const char*>(str+sizeof(str)),
                   ios, err, v);
-        assert(base(iter) == str+sizeof(str)-1);
-        assert(err == ios.goodbit);
-        assert(v == INFINITY);
+        assert(base(iter) == str);
+        assert(err == ios.failbit);
+        assert(v == 0.0l);
     }
     {
         const char str[] = "INF";
@@ -117,9 +122,9 @@ int main(int, char**)
             f.get(cpp17_input_iterator<const char*>(str),
                   cpp17_input_iterator<const char*>(str+sizeof(str)),
                   ios, err, v);
-        assert(base(iter) == str+sizeof(str)-1);
-        assert(err == ios.goodbit);
-        assert(v == INFINITY);
+        assert(base(iter) == str);
+        assert(err == ios.failbit);
+        assert(v == 0.0l);
     }
     {
         const char str[] = "-inf";
@@ -129,9 +134,9 @@ int main(int, char**)
             f.get(cpp17_input_iterator<const char*>(str),
                   cpp17_input_iterator<const char*>(str+sizeof(str)),
                   ios, err, v);
-        assert(base(iter) == str+sizeof(str)-1);
-        assert(err == ios.goodbit);
-        assert(v == -INFINITY);
+        assert(base(iter) == str + 1);
+        assert(err == ios.failbit);
+        assert(v == 0.0l);
     }
     {
         const char str[] = "-INF";
@@ -141,9 +146,9 @@ int main(int, char**)
             f.get(cpp17_input_iterator<const char*>(str),
                   cpp17_input_iterator<const char*>(str+sizeof(str)),
                   ios, err, v);
-        assert(base(iter) == str+sizeof(str)-1);
-        assert(err == ios.goodbit);
-        assert(v == -INFINITY);
+        assert(base(iter) == str + 1);
+        assert(err == ios.failbit);
+        assert(v == 0.0l);
     }
     {
         const char str[] = "nan";
@@ -153,9 +158,9 @@ int main(int, char**)
             f.get(cpp17_input_iterator<const char*>(str),
                   cpp17_input_iterator<const char*>(str+sizeof(str)),
                   ios, err, v);
-        assert(base(iter) == str+sizeof(str)-1);
-        assert(err == ios.goodbit);
-        assert(std::isnan(v));
+        assert(base(iter) == str);
+        assert(err == ios.failbit);
+        assert(v == 0.0l);
     }
     {
         const char str[] = "NAN";
@@ -165,9 +170,129 @@ int main(int, char**)
             f.get(cpp17_input_iterator<const char*>(str),
                   cpp17_input_iterator<const char*>(str+sizeof(str)),
                   ios, err, v);
-        assert(base(iter) == str+sizeof(str)-1);
-        assert(err == ios.goodbit);
-        assert(std::isnan(v));
+        assert(base(iter) == str);
+        assert(err == ios.failbit);
+        assert(v == 0.0l);
+    }
+    {
+      const char str[] = "p00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str);
+      assert(err == ios.failbit);
+      assert(v == 0.0l);
+    }
+    {
+      const char str[] = "P00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str);
+      assert(err == ios.failbit);
+      assert(v == 0.0l);
+    }
+    {
+      const char str[] = "+p00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0l);
+    }
+    {
+      const char str[] = "+P00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0l);
+    }
+    {
+      const char str[] = "-p00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0l);
+    }
+    {
+      const char str[] = "-P00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0l);
+    }
+    {
+      const char str[] = "e00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str);
+      assert(err == ios.failbit);
+      assert(v == 0.0l);
+    }
+    {
+      const char str[] = "E00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str);
+      assert(err == ios.failbit);
+      assert(v == 0.0l);
+    }
+    {
+      const char str[] = "+e00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0l);
+    }
+    {
+      const char str[] = "+E00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0l);
+    }
+    {
+      const char str[] = "-e00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0l);
+    }
+    {
+      const char str[] = "-E00";
+      std::hex(ios);
+      std::ios_base::iostate err             = ios.goodbit;
+      cpp17_input_iterator<const char*> iter = f.get(
+          cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + sizeof(str)), ios, err, v);
+      assert(base(iter) == str + 1);
+      assert(err == ios.failbit);
+      assert(v == 0.0l);
     }
     {
         const char str[] = "1.189731495357231765021264e+49321";