numpy::from_data calls vector::front and crashes (or has UB) when called with a scalar object #526

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1. mborgerding commented on Aug 12, 2026

   

   mborgerding

   on Aug 12, 2026

   Last edited by mborgerding

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   Here's an MRE using a cloned version of the function in question from ndarray.cpp (mildly edited for simplicity).
   ndarray_mre.cpp

   A compilation line for Ubuntu 24.04 at the bottom of the source file demonstrates the crash by compiling with _GLIBCXX_DEBUG and without NDEBUG:

   shape=(2,2) : array([[0.1, 0.2],
          [0.3, 0.4]], dtype=float32)
   shape=(4,) : array([0.1, 0.2, 0.3, 0.4], dtype=float32)
   /usr/include/c++/13/debug/vector:468:
   In function:
       std::debug::vector<_Tp, _Allocator>::const_reference std::
       debug::vector<_Tp, _Allocator>::front() const [with _Tp = long int; 
       _Allocator = std::allocator<long int>; const_reference = const long 
       int&]
   
   Error: attempt to access an element in an empty container.
   
   Objects involved in the operation:
       sequence "this" @ 0x7ffdcf4ae750 {
         type = std::debug::vector<long, std::allocator<long> >;
       }
   Aborted (core dumped)
   

   To see the proposed fix succeed, compile and run with -DTHE_FIX.
2. stefanseefeld

   added a commit that references this issue on Aug 24, 2026

   avoid undefined behavior:

   ...

   9b2f967