[gcc r12-7571] libstdc++: Avoid implicit narrowing from uint128_t [PR104859]

[gcc r12-7571] libstdc++: Avoid implicit narrowing from uint128_t [PR104859]

[Patrick Palka]

https://gcc.gnu.org/g:65857caee8ccfac5007a9fd0e5f18cce5e5fe934

commit r12-7571-g65857caee8ccfac5007a9fd0e5f18cce5e5fe934
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Mar 9 18:48:52 2022 -0500

    libstdc++: Avoid implicit narrowing from uint128_t [PR104859]
    
    We need to be explicit about narrowing conversions from uint128_t since,
    on targets that lack __int128, this type is defined as an integer-class
    type that is only _explicitly_ convertible to the builtin integer types.
    This issue was latent until r12-7563-ge32869a17b788b made the frontend
    correctly reject explicit conversion functions during (dependent)
    copy-initialization.
    
            PR libstdc++/104859
    
    libstdc++-v3/ChangeLog:
    
            * src/c++17/floating_to_chars.cc (__floating_to_chars_hex):
            Be explicit when narrowing the shifted effective_mantissa,
            since it may have an integer-class type.

Diff:
---
 libstdc++-v3/src/c++17/floating_to_chars.cc | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/libstdc++-v3/src/c++17/floating_to_chars.cc b/libstdc++-v3/src/c++17/floating_to_chars.cc
index 5825e661bf4..66bd457cbe2 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 = effective_mantissa >> rounded_mantissa_bits;
+	const auto nibble = unsigned(effective_mantissa >> rounded_mantissa_bits);
 	__glibcxx_assert(nibble <= 2);
 	leading_hexit = '0' + nibble;
 	effective_mantissa &= ~(mantissa_t{0b11} << rounded_mantissa_bits);
       }
     else
       {
-	const unsigned nibble = effective_mantissa >> (rounded_mantissa_bits-4);
+	const auto nibble = unsigned(effective_mantissa >> (rounded_mantissa_bits-4));
 	__glibcxx_assert(nibble < 16);
 	leading_hexit = "0123456789abcdef"[nibble];
 	effective_mantissa &= ~(mantissa_t{0b1111} << (rounded_mantissa_bits-4));
@@ -853,7 +853,7 @@ template<typename T>
 	while (effective_mantissa != 0)
 	  {
 	    nibble_offset -= 4;
-	    const unsigned nibble = effective_mantissa >> nibble_offset;
+	    const auto nibble = unsigned(effective_mantissa >> nibble_offset);
 	    __glibcxx_assert(nibble < 16);
 	    *first++ = "0123456789abcdef"[nibble];
 	    ++written_hexits;