[luatex] Hyphenation on ligatures

[Igor Liferenko]

Hi all,

The following example shows the difference when compiled with pdftex and luatex:

\language255
\righthyphenmin=2
\lefthyphenmin=2
\lccode`\'=`\'
\hyphenation{''-aa-''}
\hyphenation{bb-cc-bb}
\parindent=0pt
\hsize=1pt
\hfuzz=100pt
''aa''
bbccbb
\bye


Output of pdftex:
”aa”
bb-
cc-
bb

Output of luatex:
”-
aa-
”
bb-
cc-
bb

TeX does not not hyphenate on ligatures, even if individual characters
that generated the ligature have non-zero \lccode.

But LuaTeX hyphenates on ligatures.
Moreover, in the above example righthyphenmin/lefthyphenmin are 2, and
they must prohibit hyphenation, because ligature is one character, not
two.

Can anybody explain why and how luatex hyphenates on ligatures? Is
this incompatibility with TeX intentional, or is it just a bug which
has not been fixed?

This behavior of TeX seems quite reasonable to me and I need LuaTeX to
work the same.
Is there a workaround to make LuaTeX not hyphenate on ligatures?

Best wishes,
Igor