Deduplicate capture niches

crates/compiler/mono/src/layout.rs

@@ -1009,6 +1009,11 @@ impl<'a> LambdaSet<'a> {
                     joined.sort_by(|(lam_and_captures1, _), (lam_and_captures2, _)| {
                         lam_and_captures1.cmp(lam_and_captures2)
                     });
+                    // Remove duplicate lambda captures layouts unification can't see as
+                    // duplicates, for example [[Thunk {a: Str}, Thunk [A Str]]], each of which are
+                    // newtypes over the lambda layout `Thunk Str`.
+                    joined.dedup_by_key(|((name, captures), _)| (*name, *captures));
+
                     let (set, set_with_variables): (std::vec::Vec<_>, std::vec::Vec<_>) =
                         joined.into_iter().unzip();
 

crates/compiler/test_mono/src/tests.rs

@@ -1628,3 +1628,27 @@ fn lambda_capture_niches_have_captured_function_in_closure() {
         "#
     )
 }
+
+#[mono_test]
+fn lambda_set_niche_same_layout_different_constructor() {
+    indoc!(
+        r#"
+        capture : a -> ({} -> Str)
+        capture = \val ->
+            thunk =
+                \{} ->
+                    when val is
+                        _ -> ""
+            thunk
+
+        x : [True, False]
+        x = True
+
+        fun =
+            when x is
+                True -> capture {a: ""}
+                False -> capture (A "")
+        fun
+        "#
+    )
+}