add requirement for dims to be immutable for the prior as it is requi…

…red for the limit case masking

pymc_experimental/distributions/multivariate/r2d2m2cp.py

@@ -394,7 +394,9 @@ def R2D2M2CP(
     *broadcast_dims, dim = dims
     input_sigma = pt.as_tensor(input_sigma)
     output_sigma = pt.as_tensor(output_sigma)
-    with pm.Model(name):
+    with pm.Model(name) as model:
+        if not all(isinstance(model.dim_lengths[d], pt.TensorConstant) for d in dims):
+            raise ValueError(f"{dims!r} should be constant length imutable dims")
         if r2_std is not None:
             r2 = pm.Beta("r2", mu=r2, sigma=r2_std, dims=broadcast_dims)
         phi = _phi(

pymc_experimental/tests/distributions/test_multivariate.py

@@ -255,3 +255,27 @@ def test_zero_length_rvs_not_created(self, model: pm.Model):
             "b2", 1, [1, 1], r2=0.5, positive_probs=[1, 1], positive_probs_std=[0, 0], dims="a"
         )
         assert not model.free_RVs, model.free_RVs
+
+    def test_immutable_dims(self, model: pm.Model):
+        model.add_coord("a", range(2), mutable=True)
+        model.add_coord("b", range(2), mutable=False)
+        with pytest.raises(ValueError, match="should be constant length immutable dims"):
+            pmx.distributions.R2D2M2CP(
+                "beta0",
+                1,
+                [1, 1],
+                dims="a",
+                r2=0.8,
+                positive_probs=[0.5, 1],
+                positive_probs_std=[0.3, 0],
+            )
+        with pytest.raises(ValueError, match="should be constant length immutable dims"):
+            pmx.distributions.R2D2M2CP(
+                "beta0",
+                1,
+                [1, 1],
+                dims=("a", "b"),
+                r2=0.8,
+                positive_probs=[0.5, 1],
+                positive_probs_std=[0.3, 0],
+            )