[libcxx-commits] [libcxx] [libc++] Fix ambiguous call in {ranges, std}::find (PR #122641)

[Louis Dionne via libcxx-commits]

================
@@ -38,29 +40,45 @@ _LIBCPP_BEGIN_NAMESPACE_STD
   return __builtin_ctzll(__x);
 }
 
+// A constexpr implementation for C++11 and later (using clang extensions for constexpr support)
+// Precondition: __t != 0 (the caller __countr_zero handles __t == 0 as a special case)
 template <class _Tp>
-[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 int __countr_zero(_Tp __t) _NOEXCEPT {
-#if __has_builtin(__builtin_ctzg)
-  return __builtin_ctzg(__t, numeric_limits<_Tp>::digits);
-#else  // __has_builtin(__builtin_ctzg)
-  if (__t == 0)
-    return numeric_limits<_Tp>::digits;
-  if (sizeof(_Tp) <= sizeof(unsigned int))
+[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __countr_zero_impl(_Tp __t) _NOEXCEPT {
+  _LIBCPP_ASSERT_INTERNAL(__t != 0, "__countr_zero_impl called with zero value");
+  static_assert(is_unsigned<_Tp>::value, "__countr_zero_impl only works with unsigned types");
+  // Use constexpr if as a C++17 extension for clang
+  if _LIBCPP_CONSTEXPR (sizeof(_Tp) <= sizeof(unsigned int)) {
     return std::__libcpp_ctz(static_cast<unsigned int>(__t));
-  else if (sizeof(_Tp) <= sizeof(unsigned long))
+  } else if _LIBCPP_CONSTEXPR (sizeof(_Tp) <= sizeof(unsigned long)) {
     return std::__libcpp_ctz(static_cast<unsigned long>(__t));
-  else if (sizeof(_Tp) <= sizeof(unsigned long long))
+  } else if _LIBCPP_CONSTEXPR (sizeof(_Tp) <= sizeof(unsigned long long)) {
     return std::__libcpp_ctz(static_cast<unsigned long long>(__t));
-  else {
+  } else {
+#if _LIBCPP_STD_VER == 11
+    // A constexpr implementation for C++11 using variable declaration as a C++14 extension for clang
+    unsigned long long __ull       = static_cast<unsigned long long>(__t);
+    const unsigned int __ulldigits = numeric_limits<unsigned long long>::digits;
+    return __ull == 0ull ? __ulldigits + std::__countr_zero_impl<_Tp>(__t >> __ulldigits) : std::__libcpp_ctz(__ull);
+#else
     int __ret                      = 0;
     const unsigned int __ulldigits = numeric_limits<unsigned long long>::digits;
     while (static_cast<unsigned long long>(__t) == 0uLL) {
       __ret += __ulldigits;
       __t >>= __ulldigits;
     }
     return __ret + std::__libcpp_ctz(static_cast<unsigned long long>(__t));
+#endif
   }
-#endif // __has_builtin(__builtin_ctzg)
+}
+
+template <class _Tp>
+[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __countr_zero(_Tp __t) _NOEXCEPT {
+  static_assert(is_unsigned<_Tp>::value, "__countr_zero only works with unsigned types");
+#if __has_builtin(__builtin_ctzg)
----------------
ldionne wrote:

```suggestion
#if __has_builtin(__builtin_ctzg) // TODO(LLVM 21): This can be dropped once we only support Clang >= 19.
```

https://github.com/llvm/llvm-project/pull/122641