[petsc-dev] technical C question

[Barry Smith]

   We have code 

  return (BT_idx        = index/PETSC_BITS_PER_BYTE,
          BT_c          = array[BT_idx],
          BT_mask       = (char)(1 << index%PETSC_BITS_PER_BYTE),
          array[BT_idx] = (char)(BT_c | BT_mask),
          (char)(BT_c & BT_mask));

inside 

petscbt.h

the clang static code analyzer reports 

In file included from /sandbox/petsc/petsc.clone/include/petsc/private/matimpl.h:1233:
/sandbox/petsc/petsc.clone/include/petscbt.h:85:36: warning: The result of the '<<' expression is undefined
          BT_mask       = (char)(1 << index%PETSC_BITS_PER_BYTE),
                                 ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~

Is the code wrong/unnecessary or clang?

   Thanks