/usr/include/tcl8.6/tcl-private/generic/ is in tcl8.6-dev 8.6.1-4ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/tcl8.6/tcl-private/generic/regcustom.h
/usr/include/tcl8.6/tcl-private/generic/regerrs.h
/usr/include/tcl8.6/tcl-private/generic/regex.h
/usr/include/tcl8.6/tcl-private/generic/regguts.h
/usr/include/tcl8.6/tcl-private/generic/tcl.h
/usr/include/tcl8.6/tcl-private/generic/tclCompile.h
/usr/include/tcl8.6/tcl-private/generic/tclDecls.h
/usr/include/tcl8.6/tcl-private/generic/tclFileSystem.h
/usr/include/tcl8.6/tcl-private/generic/tclIO.h
/usr/include/tcl8.6/tcl-private/generic/tclInt.h
/usr/include/tcl8.6/tcl-private/generic/tclIntDecls.h
/usr/include/tcl8.6/tcl-private/generic/tclIntPlatDecls.h
/usr/include/tcl8.6/tcl-private/generic/tclOO.h
/usr/include/tcl8.6/tcl-private/generic/tclOODecls.h
/usr/include/tcl8.6/tcl-private/generic/tclOOInt.h
/usr/include/tcl8.6/tcl-private/generic/tclOOIntDecls.h
/usr/include/tcl8.6/tcl-private/generic/tclParse.h
/usr/include/tcl8.6/tcl-private/generic/tclPlatDecls.h
/usr/include/tcl8.6/tcl-private/generic/tclPort.h
/usr/include/tcl8.6/tcl-private/generic/tclRegexp.h
/usr/include/tcl8.6/tcl-private/generic/tclTomMath.h
/usr/include/tcl8.6/tcl-private/generic/tclTomMathDecls.h
/usr/include/tcl8.6/tcl-private/generic/tclTomMathInt.h
/usr/include/tcl8.6/tcl-private/generic/tommath.h