A parameter vector paired by a lowercase type the program declares is warned at, naming the type and where it is declared

TODO.org

@@ -854,13 +854,6 @@ Already fixed by 3672da2, which blames the arm that is not a compiler temp; the
 caret is on the last operand and =test/test_flan.ml= asserts its column. Rules
 out relabelling the else arm, a bool sentinel, and inverting the condition.
 
-** NEXT Signature pairing's cold-rebuild edge
-Decided 2026-09-25: the type takes precedence, as today. The warning is at the parameter site: where a name in a parameter vector is read as a program-declared type but could also have been read as a parameter name, the parameter vector gets a warning naming the type and where it is declared.
-Whether a parameter vector reads as one annotated parameter or two dyn ones
-depends on what type names exist, so adding a type can silently re-pair an
-existing signature between compiles. A changed-pairing warning was proposed and
-not queued.
-
 ** DONE A typed container crosses into dyn as a view, and only from permanent storage
 CLOSED: [2026-09-20]
 The descriptor is pointer, length and element type — a slice plus the piece a

lib/check.ml

@@ -1631,9 +1631,37 @@ let dyn_param_or_typo env n loc =
          parameters are lowercase"
         n
 
-let pair_params ?(also = fun _ -> false) env (items : Ast.pitem list)
+(* The pairings a parameter vector owes to a type the program declares under
-    : Ast.field list =
+   a name that is also a legal parameter name — a lowercase one, since a
+   capitalised name is refused as a parameter. [(defn f [p point] ...)] is one
+   parameter while [point] is a type and two dyn ones the moment it is not,
+   so adding or removing the type re-pairs the signature with no edit to it.
+   The type still wins; this is the warning at the parameter, filled by
+   [pair_decls] and printed by [build_program] with the other warnings. *)
+let pairing_warnings : Loc.diag list ref = ref []
+
+let pair_params ?(also = fun _ -> false) ?(declared = fun _ -> None) env
+    (items : Ast.pitem list) : Ast.field list =
   let is_type_name env n = is_type_name env n || also n in
+  let warn_pairing n t tloc =
+    let bare =
+      match String.rindex_opt t '/' with
+      | Some i -> String.sub t (i + 1) (String.length t - i - 1)
+      | None -> t
+    in
+    match declared t with
+    | Some (what, (at : Loc.t))
+      when bare <> "" && bare.[0] >= 'a' && bare.[0] <= 'z' ->
+      pairing_warnings :=
+        Loc.diag ~kind:"check/parameter-reads-a-type" tloc
+          (Printf.sprintf
+             "[%s %s] is one parameter %s of type %s, the %s declared at %s, \
+              and not two dyn parameters. If two were meant, give the second \
+              a name no type has"
+             n t n t what (Loc.to_string at))
+        :: !pairing_warnings
+    | _ -> ()
+  in
   let dyn loc = { Ast.t = Ast.Tname "dyn"; tloc = loc } in
   let rec go = function
     | [] -> []
@@ -1649,6 +1677,7 @@ let pair_params ?(also = fun _ -> false) env (items : Ast.pitem list)
     | Ast.Pname (n, loc) :: Ast.Ptype t :: rest ->
       { Ast.fname = n; fty = t; floc = loc } :: go rest
     | Ast.Pname (n, loc) :: Ast.Pname (t, tloc) :: rest when is_type_name env t ->
+      warn_pairing n t tloc;
       { Ast.fname = n; fty = { Ast.t = Ast.Tname t; tloc }; floc = loc } :: go rest
     (* The slot after this one is not a type, so this one is a parameter with
        no type written — unless the slot after it only *looks* unlike a type
@@ -1778,10 +1807,32 @@ let pair_decls env (decls : Ast.decl list) : Ast.decl list =
          match d.Ast.d with Ast.Defclass (n, _) -> Some n | _ -> None)
       decls
   in
+  (* The types the program declares, with what kind and where, for
+     [pair_params]'s warning. The prelude's are left out: its names are the
+     language's, not a declaration the reader made. *)
+  let types = Hashtbl.create 16 in
+  List.iter
+    (fun (d : Ast.decl) ->
+       let add n what =
+         if d.Ast.dloc.Loc.file <> Prelude.file then
+           Hashtbl.replace types n (what, d.Ast.dloc)
+       in
+       match d.Ast.d with
+       | Ast.Defstruct (n, _, _) -> add n "struct"
+       | Ast.Defenum (n, _) -> add n "enum"
+       | Ast.Defalias (n, _) -> add n "alias"
+       | Ast.Defdata (n, _) -> add n "data type"
+       | Ast.Defunion (n, _) -> add n "union"
+       | _ -> ())
+    decls;
+  pairing_warnings := [];
   let fn (f : Ast.fn) =
     match f.Ast.praw with
     | None -> f
-    | Some items -> { f with Ast.params = pair_params env items; praw = None }
+    | Some items ->
+      { f with
+        Ast.params = pair_params ~declared:(Hashtbl.find_opt types) env items;
+        praw = None }
   in
   List.map
     (fun (d : Ast.decl) ->
@@ -14108,6 +14159,12 @@ let build_program ~keep_going ?tolerate (decls : Ast.decl list) :
      cannot make the next body fail — which is what makes a declaration a
      resync point that needs no resynchronising. *)
   let decls = collect env decls in
+  if !print_warnings then
+    List.iter
+      (fun (d : Loc.diag) ->
+         prerr_endline
+           (Loc.entry ~mark:'~' ~label:"warning: " d.Loc.dloc d.Loc.dmsg))
+      (List.rev !pairing_warnings);
   check_finite env;
   check_union_members env;
   let s = Loc.sink ~on:keep_going in

test/test_flan.ml

@@ -5814,6 +5814,29 @@ let () =
     (match checked shadow_src with
      | _ -> true
      | exception Loc.Error _ -> false);
+  (* A parameter vector paired by a lowercase type the program declares reads
+     as two dyn parameters the day the type goes, so the pairing is warned at,
+     naming the type and where it is declared. A capitalised type cannot be a
+     parameter name, so it has nothing to warn about. *)
+  (match
+     checked "(defstruct point [x i32])\n(defstruct Vec2 [x i32])\n\
+              (defn px [p point] i32 (.x p))\n(defn vx [v Vec2] i32 (.x v))"
+   with
+   | _ ->
+     (match !Check.pairing_warnings with
+      | [ d ] ->
+        check "a lowercase declared type in a parameter vector is warned at"
+          (d.Loc.kind = "check/parameter-reads-a-type"
+           && d.Loc.dloc.Loc.line = 3 && d.Loc.dloc.Loc.col = 13
+           && d.Loc.dmsg
+              = "[p point] is one parameter p of type point, the struct \
+                 declared at <test>:1:1, and not two dyn parameters. If two \
+                 were meant, give the second a name no type has")
+      | ds ->
+        check
+          (Printf.sprintf "one pairing warning, not %d" (List.length ds))
+          false)
+   | exception Loc.Error _ -> check "the paired program checks" false);
   check "a program that shadows nothing is warned at not at all"
     (Check.shadowed_builtins (program "(defn f [] i32 1)") = []);
   (* A prelude function's name is taken over the same way, for the calls in