Make slicing type formers authoritative

Author: MaxCarroll0

src/language/statics/ConstructorStaticsHelpers.re

@@ -330,29 +330,33 @@ let ctr_uses =
   |> CoCtx.union;
 
 // A constructor pattern's payload, as a component of the type it matches.
-let payload_former = (ctr: Constructor.t): MatchedTyp.former => {
-  match_: (ctx, ty) =>
-    Typ.get_sum_constructors(ctx, ty)
-    |> Option.map(
-         List.find_map(
-           fun
-           | ConstructorMap.Variant(name, _, payload)
-               when Constructor.equal(name, ctr) =>
-             Some([payload |> Option.value(~default=Typ.gap)])
-           | _ => None,
-         ),
-       )
-    |> Option.join,
-  build:
-    fun
-    | [payload] =>
-      Sum([
-        ConstructorMap.Variant(
-          ctr,
-          ConstructorMap.empty_variant_ann,
-          Some(payload),
-        ),
-      ])
-      |> Typ.temp
-    | _ => Typ.gap,
-};
+let payload_former = (ctr: Constructor.t): MatchedTyp.former =>
+  MatchedTyp.make_former(
+    ~arity=1,
+    ~parts=
+      ty =>
+        switch (Typ.term_of(ty)) {
+        | Sum(variants) =>
+          List.find_map(
+            fun
+            | ConstructorMap.Variant(name, _, payload)
+                when Constructor.equal(name, ctr) =>
+              Some([payload |> Option.value(~default=Typ.gap)])
+            | _ => None,
+            variants,
+          )
+        | _ => None
+        },
+    ~whole=
+      fun
+      | [payload] =>
+        Sum([
+          ConstructorMap.Variant(
+            ctr,
+            ConstructorMap.empty_variant_ann,
+            Some(payload),
+          ),
+        ])
+        |> Typ.temp
+      | _ => Typ.gap,
+  );