fix: drop lengths recursively (#2775)

* fix: drop lengths recursively

* test: ensure slicing from-typetracer works

src/awkward/contents/bitmaskedarray.py

@@ -311,7 +311,7 @@ def _to_typetracer(self, forget_length: bool) -> Self:
         tt = TypeTracer.instance()
         return BitMaskedArray(
             self._mask.to_nplike(tt),
-            self._content._to_typetracer(False),
+            self._content._to_typetracer(forget_length),
             self._valid_when,
             unknown_length if forget_length else self.length,
             self._lsb_order,

src/awkward/contents/bytemaskedarray.py

@@ -234,7 +234,7 @@ def _to_typetracer(self, forget_length: bool) -> Self:
         mask = self._mask.to_nplike(tt)
         return ByteMaskedArray(
             mask.forget_length() if forget_length else mask,
-            self._content._to_typetracer(False),
+            self._content._to_typetracer(forget_length),
             self._valid_when,
             parameters=self._parameters,
         )

src/awkward/contents/indexedarray.py

@@ -224,7 +224,7 @@ def _to_typetracer(self, forget_length: bool) -> Self:
         index = self._index.to_nplike(TypeTracer.instance())
         return IndexedArray(
             index.forget_length() if forget_length else index,
-            self._content._to_typetracer(False),
+            self._content._to_typetracer(forget_length),
             parameters=self._parameters,
         )
 

src/awkward/contents/indexedoptionarray.py

@@ -213,7 +213,7 @@ def _to_typetracer(self, forget_length: bool) -> Self:
         index = self._index.to_nplike(TypeTracer.instance())
         return IndexedOptionArray(
             index.forget_length() if forget_length else index,
-            self._content._to_typetracer(False),
+            self._content._to_typetracer(forget_length),
             parameters=self._parameters,
         )
 

src/awkward/contents/listarray.py

@@ -228,7 +228,7 @@ def _to_typetracer(self, forget_length: bool) -> Self:
         return ListArray(
             starts.forget_length() if forget_length else starts,
             self._stops.to_nplike(tt),
-            self._content._to_typetracer(False),
+            self._content._to_typetracer(forget_length),
             parameters=self._parameters,
         )
 

src/awkward/contents/listoffsetarray.py

@@ -211,7 +211,7 @@ def _to_typetracer(self, forget_length: bool) -> Self:
         offsets = self._offsets.to_nplike(TypeTracer.instance())
         return ListOffsetArray(
             offsets.forget_length() if forget_length else offsets,
-            self._content._to_typetracer(False),
+            self._content._to_typetracer(forget_length),
             parameters=self._parameters,
         )
 

src/awkward/contents/unionarray.py

@@ -486,7 +486,7 @@ def _to_typetracer(self, forget_length: bool) -> Self:
         return UnionArray(
             tags.forget_length() if forget_length else tags,
             self._index.to_nplike(tt),
-            [x._to_typetracer(False) for x in self._contents],
+            [x._to_typetracer(forget_length) for x in self._contents],
             parameters=self._parameters,
         )
 

tests/test_2765_slice_from_typetracer.py

@@ -0,0 +1,15 @@
+import awkward as ak
+
+
+def test():
+    array = ak.typetracer.typetracer_from_form(
+        ak.forms.from_type(
+            ak.types.from_datashape("{x: var * {y:int64}}", highlevel=False)
+        )
+    )
+    sliced = array[0, "x", 0]
+    assert sliced.type.is_equal_to(
+        ak.types.ScalarType(
+            ak.types.RecordType([ak.types.NumpyType("int64")], ["y"]), None
+        )
+    )