[libc++][istream] P3223R2 Making std::istream::ignore less surprising

Implements [P3223R2](https://wg21.link/P3223R2)

# References

- cplusplus/draft#7996

Author: H-G-Hristov

libcxx/docs/ReleaseNotes/21.rst

@@ -51,6 +51,7 @@ Implemented Papers
 - P2441R2: ``views::join_with`` (`Github <https://github.com/llvm/llvm-project/issues/105185>`__)
 - P2711R1: Making multi-param constructors of ``views`` ``explicit`` (`Github <https://github.com/llvm/llvm-project/issues/105252>`__)
 - P2770R0: Stashing stashing ``iterators`` for proper flattening (`Github <https://github.com/llvm/llvm-project/issues/105250>`__)
+- P3223R2: Making ``std::istream::ignore`` less surprising
 
 Improvements and New Features
 -----------------------------

libcxx/include/istream

@@ -70,6 +70,7 @@ public:
     basic_istream& getline(char_type* s, streamsize n, char_type delim);
 
     basic_istream& ignore(streamsize n = 1, int_type delim = traits_type::eof());
+    basic_istream& ignore(streamsize n, char_type delim);                         // Since C++26
     int_type peek();
     basic_istream& read (char_type* s, streamsize n);
     streamsize readsome(char_type* s, streamsize n);
@@ -291,6 +292,13 @@ public:
   basic_istream& getline(char_type* __s, streamsize __n, char_type __dlm);
 
   basic_istream& ignore(streamsize __n = 1, int_type __dlm = traits_type::eof());
+#    if __LIBCPP_STD_VER >= 26
+  basic_istream& ignore(streamsize __n, char_type __delim)
+    requires is_same_v<char_type, char>
+  {
+    return ignore(__n, traits::to_int_type(__delim));
+  }
+#    endif
   int_type peek();
   basic_istream& read(char_type* __s, streamsize __n);
   streamsize readsome(char_type* __s, streamsize __n);

libcxx/test/std/input.output/iostream.format/input.streams/istream.unformatted/ignore.char_type.pass.cpp

@@ -0,0 +1,41 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++26
+
+// Requires 396145d in the built library.
+// XFAIL: using-built-library-before-llvm-9
+
+// <istream>
+
+// basic_istream& ignore(streamsize n, char_type delim);
+
+#include <cassert>
+#include <istream>
+#include <sstream>
+#include <string>
+
+#include "test_macros.h"
+
+int main(int, char**) {
+  std::istringstream in("\xF0\x9F\xA4\xA1 Clown Face");
+  in.ignore(100, '\xA1'); // ignore up to '\xA1' delimiter,
+                                   // previously might have ignored to EOF
+
+  assert(in.gcount() == 4);        // 4 bytes were ignored
+  assert(in.peek() == ' ');        // next character is a space
+
+  std::string s1;                  // read the next word
+  in >> s1;
+  assert(s1 == "Clown");
+
+  in.ignore(100, -1L);    // ambiguous overload,
+                                   // previously equivalent to (int)-1L
+
+  return 0;
+}