Fix deletion using assigning empty against arrays (fix jqlang#2051)

src/builtin.jq

@@ -11,7 +11,10 @@ def min_by(f): _min_by_impl(map([f]));
 def add: reduce .[] as $x (null; . + $x);
 def del(f): delpaths([path(f)]);
 def _assign(paths; $value): reduce path(paths) as $p (.; setpath($p; $value));
-def _modify(paths; update): reduce path(paths) as $p (.; label $out | (setpath($p; getpath($p) | update) | ., break $out), delpaths([$p]));
+def _modify(ps; f):
+  reduce path(ps) as $p
+    ([., []]; label $out | ([0] + $p as $q | setpath($q; getpath($q) | f) | ., break $out), .[1] += [$p])
+      | . as $x | $x[0] | delpaths($x[1]);
 def map_values(f): .[] |= f;
 
 # recurse

tests/jq.test

@@ -1037,6 +1037,11 @@ def inc(x): x |= .+1; inc(.[].a)
 {"a":[{"b":5}]}
 {"a":[{"c":3,"b":5}]}
 
+# #2051, deletion using assigning empty against arrays
+(.[] | select(. >= 2)) |= empty
+[1,5,3,0,7]
+[1,0]
+
 .[2][3] = 1
 [4]
 [4, null, [null, null, null, 1]]