[feaLib] "language dflt;" gets put into DFLT script

foo.fea:

languagesystem latn dflt;

feature liga {
    script latn;
    sub a by x;
} liga;

feature liga {
    script latn;
    language dflt;
    sub b by y;
} liga;

$ shape foo.ttf 'abc'
[x=0+1073|b=1+1255|c=2+975]

What?

    <ScriptList>
      <!-- ScriptCount=2 -->
      <ScriptRecord index="0">
        <ScriptTag value="DFLT"/>
        <Script>
          <DefaultLangSys>
            <ReqFeatureIndex value="65535"/>
            <!-- FeatureCount=1 -->
            <FeatureIndex index="0" value="0"/>
          </DefaultLangSys>
          <!-- LangSysCount=0 -->
        </Script>
      </ScriptRecord>
      <ScriptRecord index="1">
        <ScriptTag value="latn"/>
        <Script>
          <DefaultLangSys>
            <ReqFeatureIndex value="65535"/>
            <!-- FeatureCount=1 -->
            <FeatureIndex index="0" value="1"/>
          </DefaultLangSys>
          <!-- LangSysCount=0 -->
        </Script>
      </ScriptRecord>
    </ScriptList>
    <FeatureList>
      <!-- FeatureCount=2 -->
      <FeatureRecord index="0">
        <FeatureTag value="liga"/>
        <Feature>
          <!-- LookupCount=1 -->
          <LookupListIndex index="0" value="1"/>
        </Feature>
      </FeatureRecord>
      <FeatureRecord index="1">
        <FeatureTag value="liga"/>
        <Feature>
          <!-- LookupCount=1 -->
          <LookupListIndex index="0" value="0"/>
        </Feature>
      </FeatureRecord>
    </FeatureList>
    <LookupList>
      <!-- LookupCount=2 -->
      <Lookup index="0">
        <LookupType value="1"/>
        <LookupFlag value="0"/>
        <!-- SubTableCount=1 -->
        <SingleSubst index="0">
          <Substitution in="a" out="x"/>
        </SingleSubst>
      </Lookup>
      <Lookup index="1">
        <LookupType value="1"/>
        <LookupFlag value="0"/>
        <!-- SubTableCount=1 -->
        <SingleSubst index="0">
          <Substitution in="b" out="y"/>
        </SingleSubst>
      </Lookup>
    </LookupList>

That’s right, even though we asked for it to go into latn/dflt, it goes into DFLT/dflt! I don’t know if this is a feaLib builder bug, or a bug in the AFDKO spec, but it certainly doesn’t seem right.

(This is significant because ufo2ft feature writers write into <script>/dflt.)

[simoncozens]

Yes, it does. But this is still weird.

[simoncozens]

Okay, so maybe the problem is more a combination of this:

https://github.com/fonttools/fonttools/blob/31ba5e6b2428b088df6d0db029ac4e7d3d49bcd4/Lib/fontTools/feaLib/builder.py#L1024-L1026

and

https://github.com/fonttools/fonttools/blob/31ba5e6b2428b088df6d0db029ac4e7d3d49bcd4/Lib/fontTools/feaLib/builder.py#L1100-L1100

set_language puts the lookup into (self.script_, language), and self.script_ is set to DFLT if there is no other script statement. There may not be a DFLT script beforehand, but this combination (erroneously) creates one.