[feaLib] Emit chains to LigatureSubst even without backtrack or lookahead

https://github.com/behdad/fonttools/issues/506

Lib/fontTools/feaLib/ast.py

@@ -326,17 +326,19 @@ class LigatureCaretByPosStatement(Statement):
 
 
 class LigatureSubstStatement(Statement):
-    def __init__(self, location, prefix, glyphs, suffix, replacement):
+    def __init__(self, location, prefix, glyphs, suffix, replacement,
+                 forceChain):
         Statement.__init__(self, location)
         self.prefix, self.glyphs, self.suffix = (prefix, glyphs, suffix)
-        self.replacement = replacement
+        self.replacement, self.forceChain = replacement, forceChain
 
     def build(self, builder):
         prefix = [p.glyphSet() for p in self.prefix]
         glyphs = [g.glyphSet() for g in self.glyphs]
         suffix = [s.glyphSet() for s in self.suffix]
         builder.add_ligature_subst(
-            self.location, prefix, glyphs, suffix, self.replacement)
+            self.location, prefix, glyphs, suffix, self.replacement,
+            self.forceChain)
 
 
 class LookupFlagStatement(Statement):

Lib/fontTools/feaLib/builder.py

@@ -512,8 +512,8 @@ class Builder(object):
         self.aalt_features_.append((location, featureName))
 
     def add_ligature_subst(self, location,
-                           prefix, glyphs, suffix, replacement):
+                           prefix, glyphs, suffix, replacement, forceChain):
-        if prefix or suffix:
+        if prefix or suffix or forceChain:
             chain = self.get_lookup_(location, ChainContextSubstBuilder)
             lookup = self.get_chained_lookup_(location, LigatureSubstBuilder)
             chain.substitutions.append((prefix, glyphs, suffix, [lookup]))

Lib/fontTools/feaLib/builder_test.py

@@ -52,7 +52,7 @@ class BuilderTest(unittest.TestCase):
         spec5f_ii_1
         spec5h1 spec6b_ii spec6d2 spec6e spec6f spec6h_ii spec6h_iii_1 spec8a
         spec9b spec9c1 spec9c2 spec9c3
-        bug463 bug501 bug502 bug505
+        bug463 bug501 bug502 bug505 bug506
     """.split()
 
     def __init__(self, methodName):

Lib/fontTools/feaLib/parser.py

@@ -241,12 +241,13 @@ class Parser(object):
 
     def parse_glyph_pattern_(self, vertical):
         prefix, glyphs, lookups, values, suffix = ([], [], [], [], [])
+        hasMarks = False
         while self.next_token_ not in {"by", "from", ";"}:
             gc = self.parse_glyphclass_(accept_glyphname=True)
             marked = False
             if self.next_token_ == "'":
                 self.expect_symbol_("'")
-                marked = True
+                hasMarks = marked = True
             if marked:
                 glyphs.append(gc)
             elif glyphs:
@@ -277,18 +278,18 @@ class Parser(object):
 
         if not glyphs and not suffix:  # eg., "sub f f i by"
             assert lookups == []
-            return ([], prefix, [None] * len(prefix), values, [])
+            return ([], prefix, [None] * len(prefix), values, [], hasMarks)
         else:
             assert not any(values[:len(prefix)]), values
             values = values[len(prefix):][:len(glyphs)]
-            return (prefix, glyphs, lookups, values, suffix)
+            return (prefix, glyphs, lookups, values, suffix, hasMarks)
 
     def parse_ignore_(self):
         assert self.is_cur_keyword_("ignore")
         location = self.cur_token_location_
         self.advance_lexer_()
         if self.cur_token_ in ["substitute", "sub"]:
-            prefix, glyphs, lookups, values, suffix = \
+            prefix, glyphs, lookups, values, suffix, hasMarks = \
                 self.parse_glyph_pattern_(vertical=False)
             self.expect_symbol_(";")
             if any(lookups):
@@ -423,7 +424,7 @@ class Parser(object):
             return self.parse_position_mark_(enumerated, vertical)
 
         location = self.cur_token_location_
-        prefix, glyphs, lookups, values, suffix = \
+        prefix, glyphs, lookups, values, suffix, hasMarks = \
             self.parse_glyph_pattern_(vertical)
         self.expect_symbol_(";")
 
@@ -518,7 +519,7 @@ class Parser(object):
         assert self.cur_token_ in {"substitute", "sub", "reversesub", "rsub"}
         location = self.cur_token_location_
         reverse = self.cur_token_ in {"reversesub", "rsub"}
-        old_prefix, old, lookups, values, old_suffix = \
+        old_prefix, old, lookups, values, old_suffix, hasMarks = \
             self.parse_glyph_pattern_(vertical=False)
         if any(values):
             raise FeatureLibError(
@@ -597,7 +598,7 @@ class Parser(object):
                 num_lookups == 0):
             return ast.LigatureSubstStatement(
                 location, old_prefix, old, old_suffix,
-                list(new[0].glyphSet())[0])
+                list(new[0].glyphSet())[0], forceChain=hasMarks)
 
         # GSUB lookup type 8: Reverse chaining substitution.
         if reverse:

Lib/fontTools/feaLib/testdata/bug506.fea

@@ -0,0 +1,4 @@
+# https://github.com/behdad/fonttools/issues/506
+feature test {
+    sub f' i' by f_i;
+} test;

Lib/fontTools/feaLib/testdata/bug506.ttx

@@ -0,0 +1,66 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ttFont sfntVersion="true" ttLibVersion="3.0">
+
+  <GSUB>
+    <Version value="1.0"/>
+    <ScriptList>
+      <!-- ScriptCount=1 -->
+      <ScriptRecord index="0">
+        <ScriptTag value="DFLT"/>
+        <Script>
+          <DefaultLangSys>
+            <ReqFeatureIndex value="65535"/>
+            <!-- FeatureCount=1 -->
+            <FeatureIndex index="0" value="0"/>
+          </DefaultLangSys>
+          <!-- LangSysCount=0 -->
+        </Script>
+      </ScriptRecord>
+    </ScriptList>
+    <FeatureList>
+      <!-- FeatureCount=1 -->
+      <FeatureRecord index="0">
+        <FeatureTag value="test"/>
+        <Feature>
+          <!-- LookupCount=1 -->
+          <LookupListIndex index="0" value="0"/>
+        </Feature>
+      </FeatureRecord>
+    </FeatureList>
+    <LookupList>
+      <!-- LookupCount=2 -->
+      <Lookup index="0">
+        <!-- LookupType=6 -->
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <ChainContextSubst index="0" Format="3">
+          <!-- BacktrackGlyphCount=0 -->
+          <!-- InputGlyphCount=2 -->
+          <InputCoverage index="0">
+            <Glyph value="f"/>
+          </InputCoverage>
+          <InputCoverage index="1">
+            <Glyph value="i"/>
+          </InputCoverage>
+          <!-- LookAheadGlyphCount=0 -->
+          <!-- SubstCount=1 -->
+          <SubstLookupRecord index="0">
+            <SequenceIndex value="0"/>
+            <LookupListIndex value="1"/>
+          </SubstLookupRecord>
+        </ChainContextSubst>
+      </Lookup>
+      <Lookup index="1">
+        <!-- LookupType=4 -->
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <LigatureSubst index="0">
+          <LigatureSet glyph="f">
+            <Ligature components="i" glyph="f_i"/>
+          </LigatureSet>
+        </LigatureSubst>
+      </Lookup>
+    </LookupList>
+  </GSUB>
+
+</ttFont>