Specify reshape shape length if unknown

pytensor/tensor/shape.py

@@ -644,6 +644,8 @@ class Reshape(COp):
         x = ptb.as_tensor_variable(x)
         shp_orig = shp
         shp = ptb.as_tensor_variable(shp, ndim=1)
+        if shp.type.shape == (None,):
+            shp = specify_shape(shp, self.ndim)
         if not (
             shp.dtype in int_dtypes
             or (isinstance(shp, TensorConstant) and shp.data.size == 0)

tests/tensor/test_shape.py

@@ -98,6 +98,7 @@ class TestReshape(utt.InferShapeTester, utt.OptimizationTestMixin):
             Shape_i,
             DimShuffle,
             Elemwise,
+            SpecifyShape,
         )
         super().setup_method()
 
@@ -253,9 +254,7 @@ class TestReshape(utt.InferShapeTester, utt.OptimizationTestMixin):
             f(a_val, [7, 5])
         with pytest.raises(ValueError):
             f(a_val, [-1, -1])
-        with pytest.raises(
-            ValueError, match=".*Shape argument to Reshape has incorrect length.*"
-        ):
+        with pytest.raises(AssertionError):
             f(a_val, [3, 4, 1])
 
     def test_0(self):