[PATCH] libstdc++: Fix __max_diff_type::operator>>= for negative values

[Patrick Palka via Gcc-patches]

This patch fixes sign bit propagation when right-shifting a negative
__max_diff_type value by more than one, a bug which our test coverage
failed to uncover until r14-159-g03cebd304955a6 fixed the front end's
'signed typedef-name' handling (which is a non-standard extension to
the language grammar).

Tested on x86_64-pc-linux-gnu, does this look OK for trunk/13.2/12/11?

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
        'signed typedef-name'.  Add compile-time tests for
        right-shifting a negative __max_diff_type value by more than
        one.
---
 libstdc++-v3/include/bits/max_size_type.h             |  3 ++-
 .../testsuite/std/ranges/iota/max_size_type.cc        | 11 +++++++++--
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/libstdc++-v3/include/bits/max_size_type.h 
b/libstdc++-v3/include/bits/max_size_type.h
index 92b8168d02f..4796135d073 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 06114c22cae..985acd5a803 100644
--- a/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc
+++ b/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc
@@ -26,6 +26,11 @@
 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 = signed __int128;
+#else
+using signed_rep_t = long long;
+#endif
 
 static_assert(sizeof(max_size_t) == sizeof(max_diff_t));
 
@@ -54,6 +59,8 @@ 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 +195,7 @@ 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 +253,7 @@ 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__;
-- 
2.40.0.374.g7580f92ffa