libstdc++: Fix __max_diff_type::operator>>= for negative values

This patch fixes sign bit propagation when right-shifting a negative
__max_diff_type value by more than one, a bug that our existing test
coverage didn't expose until r14-159-g03cebd304955a6 fixed the front
end's 'signed typedef-name' handling that the test relies on (which is
a non-standard extension to the language grammar).

libstdc++-v3/ChangeLog:

	* include/bits/max_size_type.h (__max_diff_type::operator>>=):
	Fix propagation of sign bit.
	* testsuite/std/ranges/iota/max_size_type.cc: Avoid using the
	non-standard 'signed typedef-name'.  Add some compile-time tests
	for right-shifting a negative __max_diff_type value by more than
	one.

(cherry picked from commit 83470a5cd4c3d233e1d55b5e5553e1b9c553bf28)

2 files changed, 12 insertions, 3 deletions

diff --git a/libstdc++-v3/include/bits/max_size_type.h b/libstdc++-v3/include/bits/max_size_type.h
index ed421ec..da9a8a2 100644
--- a/libstdc++-v3/include/bits/max_size_type.h
+++ b/libstdc++-v3/include/bits/max_size_type.h
@@ -560,7 +560,8 @@ namespace ranges
 	// Arithmetic right shift.
 	const auto __msb = _M_rep._M_msb;
 	_M_rep >>= __r._M_rep;
-	_M_rep._M_msb |= __msb;
+	if (__msb)
+	  _M_rep |= ~(__max_size_type(-1) >> __r._M_rep);
 	return *this;
       }
 
diff --git a/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc b/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc
index 269ce29..5fc798c 100644
--- a/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc
+++ b/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc
@@ -26,8 +26,14 @@
 using max_size_t = std::ranges::__detail::__max_size_type;
 using max_diff_t = std::ranges::__detail::__max_diff_type;
 using rep_t = max_size_t::__rep;
+#if __SIZEOF_INT128__
+using signed_rep_t = __int128;
+#else
+using signed_rep_t = long long;
+#endif
 
 static_assert(sizeof(max_size_t) == sizeof(max_diff_t));
+static_assert(sizeof(rep_t) == sizeof(signed_rep_t));
 
 static_assert(std::regular<max_size_t>);
 static_assert(std::totally_ordered<max_size_t>);
@@ -54,6 +60,8 @@ test01()
   static_assert(max_diff_t(3) % -2 == 1);
   static_assert(max_diff_t(-3) << 1 == -6);
   static_assert(max_diff_t(-3) >> 1 == -2);
+  static_assert(max_diff_t(-3) >> 2 == -1);
+  static_assert(max_diff_t(-3) >> 10 == -1);
   static_assert(max_diff_t(3) >> 1 == 1);
   static_assert(max_diff_t(3) >> 2 == 0);
 
@@ -188,7 +196,7 @@ template<bool signed_p, bool shorten_p>
 void
 test02()
 {
-  using hw_type = std::conditional_t<signed_p, signed rep_t, rep_t>;
+  using hw_type = std::conditional_t<signed_p, signed_rep_t, rep_t>;
   using max_type = std::conditional_t<signed_p, max_diff_t, max_size_t>;
   using shorten_type = std::conditional_t<shorten_p, hw_type, max_type>;
   const int hw_type_bit_size = sizeof(hw_type) * __CHAR_BIT__;
@@ -246,7 +254,7 @@ template<bool signed_p, bool toggle_base_p>
 void
 test03()
 {
-  using hw_type = std::conditional_t<signed_p, signed rep_t, rep_t>;
+  using hw_type = std::conditional_t<signed_p, signed_rep_t, rep_t>;
   using max_type = std::conditional_t<signed_p, max_diff_t, max_size_t>;
   using base_type = std::conditional_t<toggle_base_p, hw_type, max_type>;
   constexpr int hw_type_bit_size = sizeof(hw_type) * __CHAR_BIT__;