diff --git a/src/extension/infer/test.rs b/src/extension/infer/test.rs
index 5714cd4e8..7653c7578 100644
--- a/src/extension/infer/test.rs
+++ b/src/extension/infer/test.rs
@@ -11,7 +11,7 @@ use crate::extension::{prelude::PRELUDE_REGISTRY, ExtensionSet};
 use crate::hugr::{validate::ValidationError, Hugr, HugrMut, HugrView, NodeType};
 use crate::macros::const_extension_ids;
 use crate::ops::custom::{ExternalOp, OpaqueOp};
-use crate::ops::{self, dataflow::IOTrait, handle::NodeHandle, OpTrait};
+use crate::ops::{self, dataflow::IOTrait, handle::NodeHandle};
 use crate::ops::{LeafOp, OpType};
 
 use crate::type_row;
@@ -314,12 +314,8 @@ fn test_conditional_inference() -> Result<(), Box<dyn Error>> {
         first_ext: ExtensionId,
         second_ext: ExtensionId,
     ) -> Result<Node, Box<dyn Error>> {
-        let [case, case_in, case_out] = create_with_io(
-            hugr,
-            conditional_node,
-            op.clone(),
-            Into::<OpType>::into(op).dataflow_signature().unwrap(),
-        )?;
+        let [case, case_in, case_out] =
+            create_with_io(hugr, conditional_node, op.clone(), op.inner_signature())?;
 
         let lift1 = hugr.add_node_with_parent(
             case,
diff --git a/src/hugr/rewrite/replace.rs b/src/hugr/rewrite/replace.rs
index 1ca02b4ac..017407c97 100644
--- a/src/hugr/rewrite/replace.rs
+++ b/src/hugr/rewrite/replace.rs
@@ -85,7 +85,7 @@ pub struct Replacement {
 }
 
 impl NewEdgeSpec {
-    fn check_src(&self, h: &impl HugrView) -> Result<(), ReplaceError> {
+    fn check_src(&self, h: &impl HugrView, err_spec: &NewEdgeSpec) -> Result<(), ReplaceError> {
         let optype = h.get_optype(self.src);
         let ok = match self.kind {
             NewEdgeKind::Order => optype.other_output() == Some(EdgeKind::StateOrder),
@@ -100,9 +100,9 @@ impl NewEdgeSpec {
             }
         };
         ok.then_some(())
-            .ok_or(ReplaceError::BadEdgeKind(Direction::Outgoing, self.clone()))
+            .ok_or_else(|| ReplaceError::BadEdgeKind(Direction::Outgoing, err_spec.clone()))
     }
-    fn check_tgt(&self, h: &impl HugrView) -> Result<(), ReplaceError> {
+    fn check_tgt(&self, h: &impl HugrView, err_spec: &NewEdgeSpec) -> Result<(), ReplaceError> {
         let optype = h.get_optype(self.tgt);
         let ok = match self.kind {
             NewEdgeKind::Order => optype.other_input() == Some(EdgeKind::StateOrder),
@@ -118,7 +118,7 @@ impl NewEdgeSpec {
             ),
         };
         ok.then_some(())
-            .ok_or(ReplaceError::BadEdgeKind(Direction::Incoming, self.clone()))
+            .ok_or_else(|| ReplaceError::BadEdgeKind(Direction::Incoming, err_spec.clone()))
     }
 
     fn check_existing_edge(
@@ -233,20 +233,20 @@ impl Rewrite for Replacement {
                     e.clone(),
                 ));
             }
-            e.check_src(h)?;
+            e.check_src(h, e)?;
         }
         self.mu_out.iter().try_for_each(|e| {
             self.replacement.valid_non_root(e.src).map_err(|_| {
                 ReplaceError::BadEdgeSpec(Direction::Outgoing, WhichHugr::Replacement, e.clone())
             })?;
-            e.check_src(&self.replacement)
+            e.check_src(&self.replacement, e)
         })?;
         // Edge targets...
         self.mu_inp.iter().try_for_each(|e| {
             self.replacement.valid_non_root(e.tgt).map_err(|_| {
                 ReplaceError::BadEdgeSpec(Direction::Incoming, WhichHugr::Replacement, e.clone())
             })?;
-            e.check_tgt(&self.replacement)
+            e.check_tgt(&self.replacement, e)
         })?;
         for e in self.mu_out.iter().chain(self.mu_new.iter()) {
             if !h.contains_node(e.tgt) || removed.contains(&e.tgt) {
@@ -256,7 +256,7 @@ impl Rewrite for Replacement {
                     e.clone(),
                 ));
             }
-            e.check_tgt(h)?;
+            e.check_tgt(h, e)?;
             // The descendant check is to allow the case where the old edge is nonlocal
             // from a part of the Hugr being moved (which may require changing source,
             // depending on where the transplanted portion ends up). While this subsumes
@@ -353,8 +353,8 @@ fn transfer_edges<'a>(
         h.valid_node(e.tgt).map_err(|_| {
             ReplaceError::BadEdgeSpec(Direction::Incoming, WhichHugr::Retained, oe.clone())
         })?;
-        e.check_src(h)?;
-        e.check_tgt(h)?;
+        e.check_src(h, oe)?;
+        e.check_tgt(h, oe)?;
         match e.kind {
             NewEdgeKind::Order => {
                 h.add_other_edge(e.src, e.tgt).unwrap();
@@ -820,7 +820,7 @@ mod test {
                 mu_out: vec![new_out_edge.clone()],
                 ..r.clone()
             }),
-            Err(ReplaceError::NoRemovedEdge(new_out_edge))
+            Err(ReplaceError::BadEdgeKind(Direction::Outgoing, new_out_edge))
         );
         Ok(())
     }
diff --git a/src/ops/controlflow.rs b/src/ops/controlflow.rs
index afd0ef5f8..4489cc4fe 100644
--- a/src/ops/controlflow.rs
+++ b/src/ops/controlflow.rs
@@ -235,10 +235,6 @@ impl OpTrait for Case {
     fn tag(&self) -> OpTag {
         <Self as StaticTag>::TAG
     }
-
-    fn dataflow_signature(&self) -> Option<FunctionType> {
-        Some(self.signature.clone())
-    }
 }
 
 impl Case {
@@ -251,6 +247,11 @@ impl Case {
     pub fn dataflow_output(&self) -> &TypeRow {
         &self.signature.output
     }
+
+    /// The signature of the dataflow sibling graph contained in the [`Case`]
+    pub fn inner_signature(&self) -> FunctionType {
+        self.signature.clone()
+    }
 }
 
 fn tuple_sum_first(tuple_sum_row: &TypeRow, rest: &TypeRow) -> TypeRow {