[Bug libstdc++/42857] std::istream::ignore(std::streamsize n) calls unnecessary underflow

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=42857

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Untested patch:

--- a/libstdc++-v3/src/c++98/compatibility.cc
+++ b/libstdc++-v3/src/c++98/compatibility.cc
@@ -88,7 +88,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
                        {
                          __sb->__safe_gbump(__size);
                          _M_gcount += __size;
-                         __c = __sb->sgetc();
+                         if (_M_gcount < __n
+                             || __n ==
__gnu_cxx::__numeric_traits<streamsize>::__max)
+                           __c = __sb->sgetc();
                        }
                      else
                        {


A similar change would be needed in the other versions of this function (the
wistream specialization, and the generic version).