[gcc r13-10083] libstdc++: Fix incorrect noexcept on string::compare overloads [PR123991]

[Jonathan Wakely]

https://gcc.gnu.org/g:96dada386d6b3417b5581c07382f54a98c75677f

commit r13-10083-g96dada386d6b3417b5581c07382f54a98c75677f
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Feb 9 10:48:44 2026 +0000

    libstdc++: Fix incorrect noexcept on string::compare overloads [PR123991]
    
    These compare overloads throw when the pos index is out of range, not
    only when the const T& parameter throws on conversion to string_view.
    
    Remove the incorrect conditional noexcept-specifier from the two
    overloads that can throw.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/123991
            * include/bits/basic_string.h (compare(size_type, size_type, T)):
            Remove noexcept-specifier.
            (compare(size_type, size_type, T, size_type, size_type)):
            Likewise.
            * include/bits/cow_string.h (compare(size_type, size_type, T)):
            Remove noexcept-specifier.
            (compare(size_type, size_type, T, size_type, size_type)):
            Likewise.
            * testsuite/21_strings/basic_string/operations/compare/char/123991.cc:
            New test.
            * testsuite/21_strings/basic_string/operations/compare/wchar_t/123991.cc:
            New test.
    
    Reviewed-by: Nathan Myers <nmyers@redhat.com>
    (cherry picked from commit 0912dfcd1e901d7dc1ff5e10528eefe3f3ff7b22)

Diff:
---
 libstdc++-v3/include/bits/basic_string.h           |  2 -
 libstdc++-v3/include/bits/cow_string.h             |  2 -
 .../basic_string/operations/compare/char/123991.cc | 56 ++++++++++++++++++++++
 .../operations/compare/wchar_t/123991.cc           | 56 ++++++++++++++++++++++
 4 files changed, 112 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 4afe9419c1fd..daadceca68ca 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -3227,7 +3227,6 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	_GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
 	_If_sv<_Tp, int>
 	compare(size_type __pos, size_type __n, const _Tp& __svt) const
-	noexcept(is_same<_Tp, __sv_type>::value)
 	{
 	  __sv_type __sv = __svt;
 	  return __sv_type(*this).substr(__pos, __n).compare(__sv);
@@ -3248,7 +3247,6 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	_If_sv<_Tp, int>
 	compare(size_type __pos1, size_type __n1, const _Tp& __svt,
 		size_type __pos2, size_type __n2 = npos) const
-	noexcept(is_same<_Tp, __sv_type>::value)
 	{
 	  __sv_type __sv = __svt;
 	  return __sv_type(*this)
diff --git a/libstdc++-v3/include/bits/cow_string.h b/libstdc++-v3/include/bits/cow_string.h
index e5f094fd13eb..af80f763c45d 100644
--- a/libstdc++-v3/include/bits/cow_string.h
+++ b/libstdc++-v3/include/bits/cow_string.h
@@ -2813,7 +2813,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       template<typename _Tp>
 	_If_sv<_Tp, int>
 	compare(size_type __pos, size_type __n, const _Tp& __svt) const
-	noexcept(is_same<_Tp, __sv_type>::value)
 	{
 	  __sv_type __sv = __svt;
 	  return __sv_type(*this).substr(__pos, __n).compare(__sv);
@@ -2833,7 +2832,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	_If_sv<_Tp, int>
 	compare(size_type __pos1, size_type __n1, const _Tp& __svt,
 		size_type __pos2, size_type __n2 = npos) const
-	noexcept(is_same<_Tp, __sv_type>::value)
 	{
 	  __sv_type __sv = __svt;
 	  return __sv_type(*this)
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/operations/compare/char/123991.cc b/libstdc++-v3/testsuite/21_strings/basic_string/operations/compare/char/123991.cc
new file mode 100644
index 000000000000..53ef2d4606c6
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/operations/compare/char/123991.cc
@@ -0,0 +1,56 @@
+// { dg-do run { target c++17 } }
+
+// Bug 123991 - std::string::compare crashes instead of throwing
+
+#include <string>
+#include <string_view>
+#include <stdexcept>
+#include <testsuite_hooks.h>
+
+void
+test_compare_3arg()
+{
+  std::string_view sv;
+  std::string s;
+  static_assert( ! noexcept(s.compare(0, 0, sv)) );
+#ifdef __cpp_exceptions
+  try
+  {
+    (void) s.compare(1, 0, sv);
+    VERIFY(false);
+  }
+  catch (const std::out_of_range&)
+  { }
+#endif
+}
+
+void
+test_compare_5arg()
+{
+  std::string_view sv;
+  std::string s;
+  static_assert( ! noexcept(s.compare(0, 0, sv, 0, 0)) );
+#ifdef __cpp_exceptions
+  try
+  {
+    (void) s.compare(1, 0, sv, 0, 0);
+    VERIFY(false);
+  }
+  catch (const std::out_of_range&)
+  { }
+
+  try
+  {
+    (void) s.compare(0, 0, sv, 1, 0);
+    VERIFY(false);
+  }
+  catch (const std::out_of_range&)
+  { }
+#endif
+}
+
+int main()
+{
+  test_compare_3arg();
+  test_compare_5arg();
+}
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/operations/compare/wchar_t/123991.cc b/libstdc++-v3/testsuite/21_strings/basic_string/operations/compare/wchar_t/123991.cc
new file mode 100644
index 000000000000..c4188bbff8b2
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/operations/compare/wchar_t/123991.cc
@@ -0,0 +1,56 @@
+// { dg-do run { target c++17 } }
+
+// Bug 123991 - std::string::compare crashes instead of throwing
+
+#include <string>
+#include <string_view>
+#include <stdexcept>
+#include <testsuite_hooks.h>
+
+void
+test_compare_3arg()
+{
+  std::wstring_view sv;
+  std::wstring s;
+  static_assert( ! noexcept(s.compare(0, 0, sv)) );
+#ifdef __cpp_exceptions
+  try
+  {
+    (void) s.compare(1, 0, sv);
+    VERIFY(false);
+  }
+  catch (const std::out_of_range&)
+  { }
+#endif
+}
+
+void
+test_compare_5arg()
+{
+  std::wstring_view sv;
+  std::wstring s;
+  static_assert( ! noexcept(s.compare(0, 0, sv, 0, 0)) );
+#ifdef __cpp_exceptions
+  try
+  {
+    (void) s.compare(1, 0, sv, 0, 0);
+    VERIFY(false);
+  }
+  catch (const std::out_of_range&)
+  { }
+
+  try
+  {
+    (void) s.compare(0, 0, sv, 1, 0);
+    VERIFY(false);
+  }
+  catch (const std::out_of_range&)
+  { }
+#endif
+}
+
+int main()
+{
+  test_compare_3arg();
+  test_compare_5arg();
+}