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 --git a/libstdc++-v3/src/c++17/floating_to_chars.cc b/libstdc++-v3/src/c++17/floating_to_chars.cc
index 5825e66..66bd457 100644 (file)
--- 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;