fix: structure removal order for unions

src/awkward/contents/unionarray.py

@@ -1574,13 +1574,23 @@ def _remove_structure(
         self, backend: Backend, options: RemoveStructureOptions
     ) -> list[Content]:
         out = []
-        for i in range(len(self._contents)):
-            index = self._index[self._tags.data == i]
-            out.extend(
-                self._contents[i]
-                ._carry(index, False)
+
+        # typetracer
+        if not self._backend.nplike.known_data:
+            self._touch_data(recursive=False)
+            # just flatten, ignore order, tags, index
+            for c in self._contents:
+                out.extend(c._remove_structure(backend, options))
+            return out
+
+        # backends with concrete data
+        for i in range(self._tags.length):
+            content = (
+                self._contents[self._tags.data[i]]
+                ._carry(ak.index.Index(self._index.data[i]), False)
                 ._remove_structure(backend, options)
             )
+            out.extend(content)
         return out
 
     def _recursively_apply(

tests/test_3180_unionarrays_order_structure_removal.py

@@ -0,0 +1,62 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/awkward/blob/main/LICENSE
+# ruff: noqa: E402
+
+from __future__ import annotations
+
+import pytest
+
+import awkward as ak
+
+
+def test_ravel_boolean():
+    x = ak.Array([None, [False], [True]])
+    y = ak.ravel(ak.fill_none(x, False, axis=0))
+    assert y.tolist() == [False, False, True]
+
+
+def test_ravel_boolean_typetracer():
+    x = ak.Array([None, [False], [True]], backend="typetracer")
+    y = ak.ravel(ak.fill_none(x, False, axis=0))
+    assert y.typestr == "## * bool"
+
+
+def test_ravel_ListOffsetArray():
+    a = ak.Array([[1, 2, 3], 4, [5, 6], 7, 8])
+    b = ak.ravel(a)
+    assert b.tolist() == [1, 2, 3, 4, 5, 6, 7, 8]
+
+
+def test_ravel_ListOffsetArray_typetracer():
+    a = ak.Array([[1, 2, 3], 4, [5, 6], 7, 8], backend="typetracer")
+    b = ak.ravel(a)
+    assert b.typestr == "## * int64"
+
+
+def test_ravel_nested_ListOffsetArrays():
+    a = ak.Array([[[1, 2], 3], 4, [5, [[[6]]]], 7, 8])
+    b = ak.ravel(a)
+    assert b.tolist() == [1, 2, 3, 4, 5, 6, 7, 8]
+
+
+def test_ravel_nested_ListOffsetArrays_typetracer():
+    a = ak.Array([[[1, 2], 3], 4, [5, [[[6]]]], 7, 8], backend="typetracer")
+    b = ak.ravel(a)
+    assert b.typestr == "## * int64"
+
+
+def test_ravel_incompatible_contents():
+    a = ak.Array([1, "high", 2, "low"])
+    with pytest.raises(
+        AssertionError,
+        match="cannot merge NumpyArray with ListOffsetArray",
+    ):
+        ak.ravel(a)
+
+
+def test_ravel_incompatible_contents_typetracer():
+    a = ak.Array([1, "high", 2, "low"], backend="typetracer")
+    with pytest.raises(
+        AssertionError,
+        match="cannot merge NumpyArray with ListOffsetArray",
+    ):
+        ak.ravel(a)