Numba IfElse: respect view flag

pytensor/link/numba/dispatch/compile_ops.py

 from copy import deepcopy
 from hashlib import sha256
+from textwrap import dedent
 
 import numba
 import numpy as np
@@ -10,6 +11,7 @@ from pytensor.compile.io import In, Out
 from pytensor.compile.mode import NUMBA
 from pytensor.compile.ops import DeepCopyOp, TypeCastingOp
 from pytensor.ifelse import IfElse
+from pytensor.link.numba.cache import compile_numba_function_src
 from pytensor.link.numba.dispatch import basic as numba_basic
 from pytensor.link.numba.dispatch.basic import (
     numba_funcify_and_cache_key,
@@ -106,30 +108,35 @@ def numba_funcify_DeepCopyOp(op, node, **kwargs):
 @register_funcify_default_op_cache_key(IfElse)
 def numba_funcify_IfElse(op, **kwargs):
     n_outs = op.n_outs
+    as_view = op.as_view
 
-    if n_outs > 1:
-
-        @numba_basic.numba_njit
-        def ifelse(cond, *args):
-            if cond:
-                res = args[:n_outs]
-            else:
-                res = args[n_outs:]
-
-            return res
+    true_names = [f"t{i}" for i in range(n_outs)]
+    false_names = [f"f{i}" for i in range(n_outs)]
+    arg_list = ", ".join((*true_names, *false_names))
 
+    if as_view:
+        true_returns = ", ".join(true_names)
     else:
+        true_returns = ", ".join(f"{name}.copy()" for name in true_names)
+    # We only ever view (alias) variables from the true branch. False branch variables must always be copied.
+    false_returns = ", ".join(f"{name}.copy()" for name in false_names)
+
+    func_src = dedent(
+        f"""
+            def ifelse(cond, {arg_list}):
+                if cond:
+                    return {true_returns}
+                else:
+                    return {false_returns}
+        """
+    )
 
-        @numba_basic.numba_njit
-        def ifelse(cond, *args):
-            if cond:
-                res = args[:n_outs]
-            else:
-                res = args[n_outs:]
-
-            return res[0]
+    ifelse_func = numba_basic.numba_njit(
+        compile_numba_function_src(func_src, "ifelse", globals())
+    )
 
-    return ifelse
+    cache_version = 1
+    return ifelse_func, cache_version
 
 
 @register_funcify_and_cache_key(CheckAndRaise)

tests/link/numba/test_compile_ops.py

 import numpy as np
 import pytest
 
-from pytensor import Mode, OpFromGraph, config, function, ifelse, scan
+from pytensor import In, Mode, OpFromGraph, Out, config, function, ifelse, scan
 from pytensor import tensor as pt
 from pytensor.compile import ViewOp
 from pytensor.graph import vectorize_graph
+from pytensor.ifelse import IfElse
 from pytensor.raise_op import assert_op
 from pytensor.scalar import Add
 from pytensor.scan.op import Scan
@@ -231,3 +232,43 @@ def test_ofg_with_inner_scan_rewrite():
     cholesky_op = scan_op.fgraph.outputs[0].owner.op
     assert isinstance(cholesky_op, Blockwise)
     assert isinstance(cholesky_op.core_op, Cholesky)
+
+
+@pytest.mark.parametrize("as_view", [True, False])
+def test_ifelse_single_output(as_view, single_out=True):
+    x = pt.vector("x")
+    y = pt.vector("y")
+    if single_out:
+        outs = [x]
+    else:
+        outs = [x, y]
+
+    op = IfElse(as_view=as_view, n_outs=len(outs))
+    outs = op(x.sum() > 0, *outs, *outs, return_list=True)
+
+    fn = function(
+        [In(x, borrow=True), In(y, borrow=True)],
+        [Out(out, borrow=True) for out in outs],
+        mode=Mode("numba", optimizer=None),
+        accept_inplace=True,
+        on_unused_input="ignore",
+    )
+
+    # FALSE branch
+    test_x = np.zeros(3)
+    test_y = np.ones(5)
+    res_false = fn(test_x, test_y)
+    for test_inp, res_out in zip([test_x, test_y], res_false, strict=False):
+        np.testing.assert_array_equal(test_inp, res_out)
+        # IfElse only views on the true branch variates
+        assert res_out is not test_inp
+
+    # TRUE branch
+    test_x = np.ones(3)
+    res_true = fn(test_x, test_y)
+    for test_inp, res_out in zip([test_x, test_y], res_true, strict=False):
+        np.testing.assert_array_equal(test_inp, res_out)
+        if as_view:
+            assert res_out is test_inp
+        else:
+            assert res_out is not test_inp