[feaLib] Behavior of using `language` without corresponding `languagesystem`

Copy-paste from a secret project issue tracker:

Me: @moyogo Cosimo says you might know why feaLib doesn’t complain about using language without a languagesystem decl 😮

@moyogo: The Feature Syntax specification isn’t clear about this. It seems to be valid to have a script or language statement that doesn’t have a matching language system declaration present.

The language system statements are to define what applies by default. The script or language statements are to specify what script and language system a feature and lookups belong to.

If a script and language statement doesn’t have a matching language system then it never has the default features and only has the features where it is specifically called. A bit odd but there might be a corner case for it.

For example

language system DFLT dflt;
language system latn CAT;
language system latn ROM;

feature feat {
# Because there is no script or language this applies to all language systems declared above
# DFLT dft, latn CAT, latn ROM but not anything else;
sub one by two;

# This only applies to latn ENG.
script latn;
language ENG;
sub one by three;
} feat;

@anthrotype: what does makeotf do in this case?

[khaledhosny]

There is no issue here, languagesystem is some syntactic sugar and does not have to be used at all (makeotf will warn if no languagesystem was used, but it that is nonsense).

From one of my fonts:

languagesystem DFLT dflt;
languagesystem arab dflt;

feature locl {
  script latn;
    sub space by space.latn;
} locl;

...

Adding languagesystem latn dflt; makes no sense as I don’t want any feature under the latn script other than locl.

[punchcutter]

If you’re using the above code then it’s going to fail because you have language system instead of languagesystem.