[gcc r15-10962] libstdc++: Add #pragma to slience null-dereference warning in istreambuf_iterator::operator++ [PR105

[Tomasz Kaminski]

https://gcc.gnu.org/g:bf06ae0fe3309e0835cbb5b21f91d7cb56126b70

commit r15-10962-gbf06ae0fe3309e0835cbb5b21f91d7cb56126b70
Author: Tomasz Kamiński <tkaminsk@redhat.com>
Date:   Fri Feb 27 19:55:19 2026 +0100

    libstdc++: Add #pragma to slience null-dereference warning in istreambuf_iterator::operator++ [PR105580]
    
    The warning was produced by following sequence, given an istream_iterator<char>
    it, such that *it will result in hitting EoF in it->_M_get(), and thus clearing
    _M_sbuf, the subsequent call to ++it, will result in _M_sbuf->sbumpc() call on
    null pointer. This is however an false-positive, as in such situation
    it == istream_iteator() returns true, and the iterator should not be
    incremented in first place.
    
    This patch sliences the issue, by disabling the "-Wnull-dereference" using
    GCC diagnostic pragmas. To work correctly the pragmas needs to be placed around
    streambuf functions on which the issue originating from istreambuf_iterator are
    reported.
    
            PR libstdc++/105580
    
    libstdc++-v3/ChangeLog:
    
            * include/std/streambuf (streambuf::gptr, streambuf::egptr)
            (streambuf::gbump): Surround with pragma disabling -Wnull-dereference.
            * testsuite/24_iterators/istreambuf_iterator/105580.cc: New test.
    
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
    Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
            (cherry picked from commits
            8758503918a91dacff4dbc7126eced21787fbfc9
            bfc2b87f8244a13ab00e8e3fe2af1d6d18fcaa36
            a523d1ecc89dcb7ea205e3de22d00443d4a0d91d)

Diff:
---
 libstdc++-v3/include/std/streambuf                        |  5 +++++
 .../testsuite/24_iterators/istreambuf_iterator/105580.cc  | 15 +++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/libstdc++-v3/include/std/streambuf b/libstdc++-v3/include/std/streambuf
index 0d0f7a826282..3b70af1ed063 100644
--- a/libstdc++-v3/include/std/streambuf
+++ b/libstdc++-v3/include/std/streambuf
@@ -490,6 +490,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       char_type*
       eback() const { return _M_in_beg; }
 
+// Required to silence false-positive warnings originating from istream_iterator::operator++,
+// see PR105580.
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wnull-dereference"
       char_type*
       gptr()  const { return _M_in_cur;  }
 
@@ -505,6 +509,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       */
       void
       gbump(int __n) { _M_in_cur += __n; }
+#pragma GCC diagnostic pop
 
       /**
        *  @brief  Setting the three read area pointers.
diff --git a/libstdc++-v3/testsuite/24_iterators/istreambuf_iterator/105580.cc b/libstdc++-v3/testsuite/24_iterators/istreambuf_iterator/105580.cc
new file mode 100644
index 000000000000..e1ef4b107396
--- /dev/null
+++ b/libstdc++-v3/testsuite/24_iterators/istreambuf_iterator/105580.cc
@@ -0,0 +1,15 @@
+// { dg-compile }
+// { dg-additional-options "-Wnull-dereference" }
+
+#include <string>
+#include <sstream>
+
+int main()
+{
+  std::istringstream in("Hello, world");
+  std::istreambuf_iterator<char> it(in), end;
+  std::string ss(it, end);
+  return 0;
+}
+
+// { dg-bogus "null pointer dereference" "" { target *-*-* } 0 }