<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/153989>153989</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            en.cppreference.com/w/cpp/memory/construct_at does not compile
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          cppljevans
      </td>
    </tr>
</table>

<pre>
    clang-20 doesn't compile when defined(USE_CONST_EVAL) in the following example:
````
//Purpose:
// Test:
/*
https://en.cppreference.com/w/cpp/memory/construct_at
 */
//===========
#include <iostream>
#include <memory>
  struct 
S 
 {
    int x;
    float y;
    double z;
  public: 
    constexpr S( int _x, float _y, double _z)
    : x(_x)
    , y(_y)
    , z(_z)
    {}
 
    [[nodiscard("no side-effects!")]]
    constexpr bool operator==(const S&) const noexcept = default;

  };//S struct

//#define USE_CONST_EVAL
#ifdef USE_CONST_EVAL
  consteval
#else
 constexpr
#endif
bool test()
{
    alignas(S) unsigned char storage[sizeof(S)]{};
    S uninitialized = std::bit_cast<S>(storage);
 std::destroy_at(&uninitialized);
    S* ptr = std::construct_at(std::addressof(uninitialized), 42, 2.71f, 3.14);
    const bool res{*ptr == S{42, 2.71f, 3.14}};
    std::destroy_at(ptr);
    return res;
}
int main() 
{
    bool const result=test();
 std::cout<<":result="<<result<<";\n";
    return 0;
} 
//=======
//Result:
/*
#ifdef USE_CONST_EVAL
  make -k --always-make run
  /usr/lib/llvm-20/bin/clang++ -c -std=c++2c -stdlib=libc++ -isystem /usr/lib/llvm-20/include/c++/v1   -ftemplate-backtrace-limit=0 -g     -c construct_at.constexpr.cpp -MMD -o build/boost-1_85_0/clangxx-20-2c/construct_at.constexpr.o 
 construct_at.constexpr.cpp:45:23: error: call to consteval function 'test' is not a constant expression
     45 |     bool const result=test();
        | ^
  construct_at.constexpr.cpp:36:7: note: destroying object 'uninitialized' whose lifetime has already ended
     36 |     S uninitialized = std::bit_cast<S>(storage);
        |       ^
 construct_at.constexpr.cpp:45:23: note: in call to 'test()'
     45 | bool const result=test();
        |                       ^~~~~~
  1 error generated.
  make: *** [/home/larry/prog_dev/Makefile.incs/MAIN.imk:10: build/boost-1_85_0/clangxx-20-2c/construct_at.constexpr.o] Error 1
  make: Target 'run' not remade because of errors.
  
  Compilation exited abnormally with code 2 at Sat Aug 16 20:22:47, duration 0.48 s
#else
  make -k --always-make run
  /usr/lib/llvm-20/bin/clang++ -c -std=c++2c -stdlib=libc++ -isystem /usr/lib/llvm-20/include/c++/v1 -ftemplate-backtrace-limit=0 -g     -c   construct_at.constexpr.cpp -MMD -o build/boost-1_85_0/clangxx-20-2c/construct_at.constexpr.o 
  sed -e 's#^construct_at.constexpr.o:#build/boost-1_85_0/clangxx-20-2c/construct_at.constexpr.o:#' ./build/boost-1_85_0/clangxx-20-2c/construct_at.constexpr.d > build/boost-1_85_0/clangxx-20-2c/construct_at.constexpr.o.dep
  rm ./build/boost-1_85_0/clangxx-20-2c/construct_at.constexpr.d
  install -d build/boost-1_85_0/clangxx-20-2c
  /usr/lib/llvm-20/bin/clang++ -L/usr/lib/llvm-20/lib -Wl,-rpath,/usr/lib/llvm-20/lib -lc++ -lc++abi  -v -fuse-ld=lld    build/boost-1_85_0/clangxx-20-2c/construct_at.constexpr.o  -o build/boost-1_85_0/clangxx-20-2c/construct_at.constexpr.exe
  Ubuntu clang version 20.1.8 (++20250804090239+87f0227cb601-1~exp1~20250804210352.139)
 Target: x86_64-pc-linux-gnu
  Thread model: posix
  InstalledDir: /usr/lib/llvm-20/bin
  Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/14
  Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/11
  Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/12
  Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/13
  Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/14
  Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/14
  Candidate multilib: .;@m64
  Selected multilib: .;@m64
   "/usr/lib/llvm-20/bin/ld.lld" -z relro --hash-style=gnu --build-id --eh-frame-hdr -m elf_x86_64 -pie -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o build/boost-1_85_0/clangxx-20-2c/construct_at.constexpr.exe /lib/x86_64-linux-gnu/Scrt1.o /lib/x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/14/crtbeginS.o -L/usr/lib/llvm-20/lib -L/usr/lib/gcc/x86_64-linux-gnu/14 -L/usr/lib/gcc/x86_64-linux-gnu/14/../../../../lib64 -L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib64 -L/usr/lib/llvm-20/bin/../lib -L/lib -L/usr/lib -rpath /usr/lib/llvm-20/lib -lc++ -lc++abi build/boost-1_85_0/clangxx-20-2c/construct_at.constexpr.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/lib/gcc/x86_64-linux-gnu/14/crtendS.o /lib/x86_64-linux-gnu/crtn.o
 ./build/boost-1_85_0/clangxx-20-2c/construct_at.constexpr.exe 
 :result=1;
  
  Compilation finished at Sat Aug 16 20:27:34, duration 0.50 s
#endif//USE_CONST_EVAL
 */ 
````
I downloaded llvm-20 from synatic.  My system is revealed by:

````
larry@larry-Precision-3581:~/prog_dev/Makefile.incs$ uname -a
Linux larry-Precision-3581 6.5.0-1027-oem #28-Ubuntu SMP PREEMPT_DYNAMIC Thu Jul 25 13:32:46 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux
````
</pre>
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