[libc++] Implement LWG3464(istream::gcount() can overflow)

Implement LWG3464 https://wg21.link/LWG3464

Reviewed By: #libc, Mordante, philnik

Differential Revision: https://reviews.llvm.org/D158749

Author: yronglin

libcxx/docs/Status/Cxx23Issues.csv

@@ -10,7 +10,7 @@
 "`3432 <https://wg21.link/LWG3432>`__","Missing requirement for ``comparison_category``","November 2020","|Complete|","16.0","|spaceship|"
 "`3447 <https://wg21.link/LWG3447>`__","Deduction guides for ``take_view`` and ``drop_view`` have different constraints","November 2020","|Complete|","14.0","|ranges|"
 "`3450 <https://wg21.link/LWG3450>`__","The const overloads of ``take_while_view::begin/end`` are underconstrained","November 2020","|Complete|","16.0","|ranges|"
-"`3464 <https://wg21.link/LWG3464>`__","``istream::gcount()`` can overflow","November 2020","",""
+"`3464 <https://wg21.link/LWG3464>`__","``istream::gcount()`` can overflow","November 2020","|Complete|","18.0"
 "`2731 <https://wg21.link/LWG2731>`__","Existence of ``lock_guard<MutexTypes...>::mutex_type`` typedef unclear","November 2020","|Complete|","5.0"
 "`2743 <https://wg21.link/LWG2743>`__","P0083R3 ``node_handle`` private members missing ""exposition only"" comment","November 2020","|Nothing To Do|",""
 "`2820 <https://wg21.link/LWG2820>`__","Clarify ``<cstdint>`` macros","November 2020","|Nothing To Do|",""

libcxx/include/istream

@@ -185,6 +185,11 @@ class _LIBCPP_TEMPLATE_VIS basic_istream
     : virtual public basic_ios<_CharT, _Traits>
 {
     streamsize __gc_;
+
+    _LIBCPP_HIDE_FROM_ABI void __inc_gcount() {
+      if (__gc_ < numeric_limits<streamsize>::max())
+        ++__gc_;
+    }
 public:
     // types (inherited from basic_ios (27.5.4)):
     typedef _CharT                         char_type;
@@ -714,7 +719,7 @@ basic_istream<_CharT, _Traits>::operator>>(basic_streambuf<char_type, traits_typ
                             __sb->sputc(traits_type::to_char_type(__i)),
                             traits_type::eof()))
                         break;
-                    ++__gc_;
+                    __inc_gcount();
                     this->rdbuf()->sbumpc();
                 }
                 if (__gc_ == 0)
@@ -806,7 +811,7 @@ basic_istream<_CharT, _Traits>::get(char_type* __s, streamsize __n, char_type __
                     if (traits_type::eq(__ch, __dlm))
                         break;
                     *__s++ = __ch;
-                    ++__gc_;
+                    __inc_gcount();
                      this->rdbuf()->sbumpc();
                 }
                 if (__gc_ == 0)
@@ -867,7 +872,7 @@ basic_istream<_CharT, _Traits>::get(basic_streambuf<char_type, traits_type>& __s
                     break;
                 if (traits_type::eq_int_type(__sb.sputc(__ch), traits_type::eof()))
                     break;
-                ++__gc_;
+                __inc_gcount();
                 this->rdbuf()->sbumpc();
             }
 #ifndef _LIBCPP_HAS_NO_EXCEPTIONS
@@ -910,7 +915,7 @@ basic_istream<_CharT, _Traits>::getline(char_type* __s, streamsize __n, char_typ
                 if (traits_type::eq(__ch, __dlm))
                 {
                     this->rdbuf()->sbumpc();
-                    ++__gc_;
+                    __inc_gcount();
                     break;
                 }
                 if (__gc_ >= __n-1)
@@ -920,7 +925,7 @@ basic_istream<_CharT, _Traits>::getline(char_type* __s, streamsize __n, char_typ
                 }
                 *__s++ = __ch;
                 this->rdbuf()->sbumpc();
-                ++__gc_;
+                __inc_gcount();
             }
 #ifndef _LIBCPP_HAS_NO_EXCEPTIONS
         }
@@ -970,7 +975,7 @@ basic_istream<_CharT, _Traits>::ignore(streamsize __n, int_type __dlm)
                        __state |= ios_base::eofbit;
                        break;
                     }
-                    ++__gc_;
+                    __inc_gcount();
                     if (traits_type::eq_int_type(__i, __dlm))
                         break;
                 }
@@ -985,7 +990,7 @@ basic_istream<_CharT, _Traits>::ignore(streamsize __n, int_type __dlm)
                        __state |= ios_base::eofbit;
                        break;
                     }
-                    ++__gc_;
+                    __inc_gcount();
                     if (traits_type::eq_int_type(__i, __dlm))
                         break;
                 }