[ChainContext] Fix fast-path deviation from slow path

See comments. The input skippy was hitting the U+200D and NOT
seeing it skippable, whereas the context skippy would had.

Fixes https://github.com/harfbuzz/harfbuzz/issues/4813

src/hb-ot-layout-gsubgpos.hh

@@ -3390,6 +3390,15 @@ struct ChainRuleSet
      *
      * Replicated from LigatureSet::apply(). */
 
+    /* If the input skippy has non-auto joiners behavior (as in Indic shapers),
+     * skip this fast path, as we don't distinguish between input & lookahead
+     * matching in the fast path.
+     *
+     * https://github.com/harfbuzz/harfbuzz/issues/4813
+     */
+    if (!c->auto_zwnj || !c->auto_zwj)
+      goto slow;
+
     hb_ot_apply_context_t::skipping_iterator_t &skippy_iter = c->iter_input;
     skippy_iter.reset (c->buffer->idx);
     skippy_iter.set_match_func (match_always, nullptr);
@@ -3429,10 +3438,10 @@ struct ChainRuleSet
     }
     matched = skippy_iter.next ();
     if (likely (matched && !skippy_iter.may_skip (c->buffer->info[skippy_iter.idx])))
-     {
+    {
       second = &c->buffer->info[skippy_iter.idx];
       unsafe_to2 = skippy_iter.idx + 1;
-     }
+    }
 
     auto match_input = lookup_context.funcs.match[1];
     auto match_lookahead = lookup_context.funcs.match[2];

test/shape/data/in-house/tests/indic-joiners.tests

@@ -4,3 +4,4 @@
 ../fonts/8116e5d8fedfbec74e45dc350d2416d810bed8c4.ttf;;U+091F,U+094D,U+200D,U+092F,U+093F;[uni093F=0+398|uni091F=0+876|uni094D=0@4,0+0|space=0+0|uni092F=0+924]
 ../fonts/8116e5d8fedfbec74e45dc350d2416d810bed8c4.ttf;;U+091F,U+094D,U+200D,U+091F,U+094D,U+200C,U+091F,U+094D,U+200D,U+092F,U+093F;[uni091F=0+876|uni094D=0@4,0+0|space=0+0|uni091F=3+876|uni094D=3@4,0+0|space=5+0|uni093F=6+398|uni091F=6+876|uni094D=6@4,0+0|space=6+0|uni092F=6+924]
 ../fonts/8116e5d8fedfbec74e45dc350d2416d810bed8c4.ttf;;U+091F,U+094D,U+200D,U+091F,U+094D,U+200D,U+091F,U+094D,U+200D,U+092F,U+093F;[uni093F=0+398|uni091F=0+876|uni094D=0@4,0+0|space=0+0|uni091F=0+876|uni094D=0@4,0+0|space=0+0|uni091F=0+876|uni094D=0@4,0+0|space=0+0|uni092F=0+924]
+../fonts/63e224dcb3d559d590f80c83b832cfca789e5dcc.ttf;;U+0ABE,U+0AA8,U+0ACD,U+200D,U+0AA4,U+0ABF;[uni25CC=0+596|uni0ABE=0+251|uni0ABF.05=1+251|uni0AA80ACD=1+293|space=1+0|uni0AA4=1+543]