Library /sys$common/syshlp/basichelp.hlb Compile time errors, OVFCHKSUP  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  ERROR - Overflow checking was specified for a floating-point data type
  in  a  compiler  command,  a qualifier to the DCL BASIC command, or an
  OPTION statement.  Specify overflow checking only for  INTEGER  and/or
  DECIMAL data types.