From 29291357f11ba6f81961a0cb3e1035d7493b1c3e Mon Sep 17 00:00:00 2001
From: Alan Lawrence <alan.lawrence@cambridgequantum.com>
Date: Tue, 14 May 2024 12:50:38 +0100
Subject: [PATCH] Add Type::row_var_bound

---
 hugr/src/types.rs            |  8 ++++++++
 hugr/src/types/type_param.rs | 16 ++++++++--------
 2 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/hugr/src/types.rs b/hugr/src/types.rs
index e763bad12..78a407e1c 100644
--- a/hugr/src/types.rs
+++ b/hugr/src/types.rs
@@ -351,6 +351,14 @@ impl Type {
         TypeBound::Copyable.contains(self.least_upper_bound())
     }
 
+    /// If this Type is a row variable, return its bound, otherwise None
+    pub fn row_var_bound(&self) -> Option<TypeBound> {
+        match self.0 {
+            TypeEnum::RowVariable(_, b) => Some(b),
+            _ => None,
+        }
+    }
+
     /// Checks that this [Type] represents a single Type, not a row variable,
     /// that all variables used within are in the provided list of bound variables,
     /// and that for each [CustomType], the corresponding
diff --git a/hugr/src/types/type_param.rs b/hugr/src/types/type_param.rs
index 92daa58ab..9cfba9852 100644
--- a/hugr/src/types/type_param.rs
+++ b/hugr/src/types/type_param.rs
@@ -12,7 +12,6 @@ use crate::extension::ExtensionRegistry;
 use crate::extension::ExtensionSet;
 use crate::extension::SignatureError;
 
-use super::TypeEnum;
 use super::{check_typevar_decl, CustomType, Substitution, Type, TypeBound};
 
 /// The upper non-inclusive bound of a [`TypeParam::BoundedNat`]
@@ -366,7 +365,7 @@ fn check_type_arg_rv(
         ) if param.contains(cached_decl) => Ok(()),
         (TypeArg::Type { ty }, TypeParam::Type { b: bound })
             if bound.contains(ty.least_upper_bound())
-                && (allow_rowvars || !matches!(ty.0, TypeEnum::RowVariable(_, _))) =>
+                && (allow_rowvars || ty.row_var_bound().is_none()) =>
         {
             Ok(())
         }
@@ -378,12 +377,13 @@ fn check_type_arg_rv(
         }
         // Also allow a single "Type" to be used for a List *only* if the Type is a row variable
         // (i.e., it's not really a Type, it's multiple Types)
-        (
-            TypeArg::Type {
-                ty: Type(TypeEnum::RowVariable(_, b), _),
-            },
-            TypeParam::List { param },
-        ) if param.contains(&(*b).into()) => Ok(()),
+        (TypeArg::Type { ty }, TypeParam::List { param })
+            if ty
+                .row_var_bound()
+                .is_some_and(|b| param.contains(&b.into())) =>
+        {
+            Ok(())
+        }
 
         (TypeArg::Sequence { elems: items }, TypeParam::Tuple { params: types }) => {
             if items.len() != types.len() {