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Showing content from https://github.com/llvm/llvm-project/issues/55638 below:

std::is_constant_evaluated returns true in C++2b in manifestly runtime functions · Issue #55638 · llvm/llvm-project · GitHub

Consider the code:

#include <type_traits>
#include <cmath>
#include <iostream>

template <class Integer>
constexpr Integer bitwise_sqrt(const Integer& x)
{
   // TODO put real code here, doesn't need it for the bug case though
   return 0;
}
template <class Integer>
constexpr Integer fast_sqrt(const Integer& x)
{
   return static_cast<Integer>(std::sqrt(static_cast<double>(x)));
}
template <class Integer>
constexpr Integer get_sqrt(const Integer& x)
{
  if(std::is_constant_evaluated())
    return bitwise_sqrt(x);
   return fast_sqrt(x);
}


int main()
{
  unsigned i;
  std::cin >> i;
  i = get_sqrt(i);
  std::cout << i << std::endl;
}

Input comes from std::cin, I assume we can all agree that in this context get_sqrt() can not be a constant context. However, std::is_constant_evaluated() is returning true.

Note that:

1. The issue only occurs when compiling with -std=c++2b.
2. The issue does not occur prior to clang-14.
3. The issue does not occur if we use __builtin_is_constant_evaluated() directly.
4. Reproduced on Ubuntu-22 with ubuntu clang version 14.0.0-1ubuntu1

This is a reduced test case from Boost.Multiprecision BTW.

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