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

[H-G-Hristov]

Implements P3223R2 as DR as recommended. Resolves -1L ambiguity.

Closes #148178

References

• P3223R2 Making std::istream::ignore less surprising cplusplus/draft#7996
• istream.general
• istream.unformatted
• std::istream::ignore
• DR recommendation

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Hristo Hristov (H-G-Hristov)

Changes

Implements P3223R2

References

• P3223R2 Making std::istream::ignore less surprising cplusplus/draft#7996

---

Full diff: https://github.com/llvm/llvm-project/pull/147007.diff

4 Files Affected:

• (modified) libcxx/docs/ReleaseNotes/21.rst (+1)
• (modified) libcxx/include/istream (+9)
• (added) libcxx/test/std/input.output/iostream.format/input.streams/istream.unformatted/ignore.char_type.pass.cpp (+37)
• (added) libcxx/test/std/input.output/iostream.format/input.streams/istream.unformatted/ignore.verify.cpp (+27)

diff --git a/libcxx/docs/ReleaseNotes/21.rst b/libcxx/docs/ReleaseNotes/21.rst
index 08b32bb508dc1..34735ca515d53 100644
--- a/libcxx/docs/ReleaseNotes/21.rst
+++ b/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
 -----------------------------
diff --git a/libcxx/include/istream b/libcxx/include/istream
index 02546902494e3..83ba1799cae91 100644
--- a/libcxx/include/istream
+++ b/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);
@@ -171,6 +172,7 @@ template <class Stream, class T>
 #    include <__type_traits/conjunction.h>
 #    include <__type_traits/enable_if.h>
 #    include <__type_traits/is_base_of.h>
+#    include <__type_traits/is_same.h>
 #    include <__type_traits/make_unsigned.h>
 #    include <__utility/declval.h>
 #    include <__utility/forward.h>
@@ -291,6 +293,13 @@ 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
+  _LIBCPP_HIDE_FROM_ABI basic_istream& ignore(streamsize __n, char_type __delim)
+    requires is_same_v<char_type, char>
+  {
+    return this->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);
diff --git a/libcxx/test/std/input.output/iostream.format/input.streams/istream.unformatted/ignore.char_type.pass.cpp b/libcxx/test/std/input.output/iostream.format/input.streams/istream.unformatted/ignore.char_type.pass.cpp
new file mode 100644
index 0000000000000..9d83a7ea15027
--- /dev/null
+++ b/libcxx/test/std/input.output/iostream.format/input.streams/istream.unformatted/ignore.char_type.pass.cpp
@@ -0,0 +1,37 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 <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 str; // read the next word
+  in >> str;
+  assert(str == "Clown");
+
+  return 0;
+}
diff --git a/libcxx/test/std/input.output/iostream.format/input.streams/istream.unformatted/ignore.verify.cpp b/libcxx/test/std/input.output/iostream.format/input.streams/istream.unformatted/ignore.verify.cpp
new file mode 100644
index 0000000000000..54d13cac69b94
--- /dev/null
+++ b/libcxx/test/std/input.output/iostream.format/input.streams/istream.unformatted/ignore.verify.cpp
@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 <sstream>
+#include <string>
+
+#include "test_macros.h"
+
+void test() {
+  std::istringstream in("\xF0\x9F\xA4\xA1 Clown Face");
+  in.ignore(100, -1L); // expected-error {{call to member function 'ignore' is ambiguous}}
+}

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[ldionne]

LGTM assuming @frederick-vs-ja is happy

[ldionne]

Sorry, I missed something subtle during my first review

[philnik777]

Why is this implemented as a DR?

[frederick-vs-ja]

Why is this implemented as a DR?

The reason is available in https://github.com/cplusplus/papers/issues/1871#issuecomment-2993018698.

[frederick-vs-ja]

Looks like review comments are resolved. LGTM with conflicts in Cxx2cPapers.csv resolved.

[Zingam]

@ldionne gentle ping

[frederick-vs-ja]

@ldionne Ping. I think we can land this.