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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/104859] [12 regression] liibg++ fails during bootstrap
Date: Wed, 09 Mar 2022 22:09:04 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104859

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot g=
nu.org
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2022-03-09

--- Comment #5 from Patrick Palka <ppalka at gcc dot gnu.org> ---
I think we just need to add some explicit casts:

diff --git a/libstdc++-v3/src/c++17/floating_to_chars.cc
b/libstdc++-v3/src/c++17/floating_to_chars.cc
index 5825e661bf4..0ccf2623511 100644
--- a/libstdc++-v3/src/c++17/floating_to_chars.cc
+++ b/libstdc++-v3/src/c++17/floating_to_chars.cc
@@ -801,14 +801,14 @@ template<typename T>
     char leading_hexit;
     if constexpr (has_implicit_leading_bit)
       {
-       const unsigned nibble =3D effective_mantissa >> rounded_mantissa_bi=
ts;
+       const unsigned nibble =3D unsigned(effective_mantissa >>
rounded_mantissa_bits);
        __glibcxx_assert(nibble <=3D 2);
        leading_hexit =3D '0' + nibble;
        effective_mantissa &=3D ~(mantissa_t{0b11} << rounded_mantissa_bits=
);
       }
     else
       {
-       const unsigned nibble =3D effective_mantissa >>
(rounded_mantissa_bits-4);
+       const unsigned nibble =3D unsigned(effective_mantissa >>
(rounded_mantissa_bits-4));
        __glibcxx_assert(nibble < 16);
        leading_hexit =3D "0123456789abcdef"[nibble];
        effective_mantissa &=3D ~(mantissa_t{0b1111} <<
(rounded_mantissa_bits-4));
@@ -853,7 +853,7 @@ template<typename T>
        while (effective_mantissa !=3D 0)
          {
            nibble_offset -=3D 4;
-           const unsigned nibble =3D effective_mantissa >> nibble_offset;
+           const unsigned nibble =3D unsigned(effective_mantissa >>
nibble_offset);
            __glibcxx_assert(nibble < 16);
            *first++ =3D "0123456789abcdef"[nibble];
            ++written_hexits;

since on targets which lack __int128, effective_mantissa can be the
integer-class uint128_t, which unlike __int128 is only _explicitly_ convert=
ible
to the builtin integer types.   And r12-7563-ge32869a17b788b made us correc=
tly
exclude explicit conversion operators during copy-initialization.=