A union is not a missing struct either

layout searched only Tast.structs, so a declared union came back as "no struct
is named X" — which reads as "that type does not exist" about a type the
checker knows. Refused by kind beside the enum, and both refusals now have a
test: a new enum and a new union, evaluated into the session.

BUILT.md

@@ -1042,7 +1042,9 @@ suffix would reintroduce the ambiguity the rule exists to remove, and a rule wit
 rely on. The refusal carries `:candidates`, so a person is one copy-paste from the answer and a client has its
 completion list — the same shape as `package_of` refusing a directory imported under two aliases rather than picking
 one. An enum is refused by *kind* (`X is an enum, not a struct`): its members are erased to `i32` before `Tast.program`
-exists, which is the same fact that makes a `defenum` unreloadable.
+exists, which is the same fact that makes a `defenum` unreloadable. A union is refused the same way and for its own
+reason — it is declared, and union *values* are milestone 6. Both are `Types.Named` at a use site, so falling through
+to "no struct is named X" would say a type does not exist about one that plainly does.
 
 **`render.ml` is not reused, and that is not a second walk.** It walks a *value* and emits the code that prints it;
 this describes a *type* and emits text. What is shared is the spelling: field types go through `Types.to_string`, which

lib/dev.ml

@@ -529,12 +529,18 @@ let layout t ~ty =
                       Wire.quote (Types.to_string f.Tast.fty) ])
                s.Tast.fields) ]
   | None ->
-    (* An enum is a type the checker knows and this op cannot describe: its
+    (* Two types the checker knows and this op cannot describe. An enum's
        members are erased to i32 before [Tast.program] exists, which is the
-       same fact that makes a defenum unreloadable. Saying which kind it is
-       beats "no such type" for a name that plainly exists. *)
+       same fact that makes a defenum unreloadable; a union is declared and
+       has no values yet. Either way, saying which kind it is beats "no such
+       type" for a name that plainly exists. *)
     if Hashtbl.mem t.session.Session.env.Check.enums ty then
       error (ty ^ " is an enum, not a struct; its members are erased to i32")
+    else if
+      List.exists (fun (u : Tast.union) -> String.equal u.Tast.uname ty)
+        t.session.Session.program.Tast.unions
+    then
+      error (ty ^ " is a union, not a struct; union values are milestone 6")
     else
       let suffix = "/" ^ ty in
       let candidates =

test/test_dev.ml

@@ -174,6 +174,44 @@ let () =
       let r = request c "(:op \"layout\" :type \"Nonesuch\")" in
       if status r <> "error" then fail "a type that does not exist got a layout";
 
+      (* A name that plainly exists and is not a struct is refused by *kind*.
+         Both of these are types the checker knows and this op cannot
+         describe, and "no struct is named X" would read as "X does not
+         exist". *)
+      let refusal r =
+        Option.value ~default:(status r) (Wire.string_field r "message")
+      in
+      let contains hay needle =
+        let n = String.length needle in
+        let rec go i =
+          i + n <= String.length hay
+          && (String.equal (String.sub hay i n) needle || go (i + 1))
+        in
+        go 0
+      in
+      let r =
+        request c
+          "(:op \"eval\" :code \"(defenum Colour [red 0 green 1])\" :file \"/tmp/buf.flan\")"
+      in
+      if status r <> "ok" then fail "a new enum: %s" (refusal r)
+      else begin
+        let r = request c "(:op \"layout\" :type \"Colour\")" in
+        if status r <> "error" then fail "an enum answered a struct layout"
+        else if not (contains (refusal r) "is an enum") then
+          fail "an enum is refused as: %s" (refusal r)
+      end;
+      let r =
+        request c
+          "(:op \"eval\" :code \"(defunion Shape [(Circle [r f32])])\" :file \"/tmp/buf.flan\")"
+      in
+      if status r <> "ok" then fail "a new union: %s" (refusal r)
+      else begin
+        let r = request c "(:op \"layout\" :type \"Shape\")" in
+        if status r <> "error" then fail "a union answered a struct layout"
+        else if not (contains (refusal r) "is a union") then
+          fail "a union is refused as: %s" (refusal r)
+      end;
+
       (* The identity rule, and the case NEXT.md named: a second [Blob] typed
          into a package is [agent/Blob], the qualified name resolves, and the
          bare one is refused with the names it could have meant rather than