[committed] libstdc++: Avoid implicit narrowing from uint128_t [PR104859]

[committed] libstdc++: Avoid implicit narrowing from uint128_t [PR104859]

[Patrick Palka]

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.

Tested on x86_64-pc-linux-gnu using both possible definitions of uint128
(as an alias for unsigned __int128 and as the integer-class type), committed to
trunk as obvious.

	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.
---
 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;
-- 
2.35.1.415.gc2162907e9