A number or character literal bound by let or loop takes its type from its uses in the function

TODO.org

@@ -33,6 +33,9 @@ disagree are refused with a request for an annotation. Vector, map and text lite
 are dyn unless something typed wants them. A typed value is boxed where it goes into
 dyn, and a dyn unboxed (checked) where typed code needs it; typed beside dyn in an
 operator gives dyn. Dyn integers stay i64 and dyn floats f64.
+Local inference is in (check.ml [lit_session]). The f32 default and dyn text and vector
+literals wait on the author's answers to the phase-1 measurements; =FLAN_LIT=f32,dyn= in
+check.ml is the measuring switch, to be deleted with them.
 ** DONE Dynamic-first, and the dyn half of the language
 CLOSED: [2026-09-20]
 An unannotated parameter or return is =dyn=: a NaN-boxed value over a mark-sweep

lib/check.ml

@@ -70,6 +70,10 @@ type binding = {
      that the field is already in hand. [None] everywhere else, and a refusal
      with [None] says exactly what it said before. *)
   bwhat : string option;
+  (* The literal a [let] or [loop] bound this name to, when its type is read
+     off the uses ([lit_session]). The initialiser's node, by identity, is the
+     key: two expansions of one macro are two nodes. *)
+  blit : Ast.expr option;
 }
 
 (* A class slot's type: what a value stored into it is checked against. A
@@ -725,11 +729,53 @@ type lentry =
   | Lrecur   of (int * Types.t) list
   | Lbarrier of string
 
+(* Local inference for a number or character literal bound by a [let] or a
+   [loop]: [(let [t 0.0] ... (set t (+ t x)))] makes [t] x's type. The uses
+   are read by checking the form once with the literal locals at their current
+   guess and every hook below recording instead of refusing, then undoing that
+   check; the guesses are solved, and the form is checked for real. A guess
+   that moved is checked again, at most [lit_rounds] times, so a local fed by
+   another settles. One session per function context, opened by its outermost
+   such [let], so a lambda or a generic's copy is inferred on its own and a
+   literal's type never depends on another function.
+
+   What a use says about the local:
+   - [Up t]: the local flows into a [t] — a parameter, a return, a field, an
+     index. The local has to widen into [t].
+   - [Down t]: a [t] is [set] into it, or passed to [recur] for it. [t] has to
+     widen into the local.
+   - [Hint t]: an operator's other operand, which meets it at either.
+   A [set] of one such local into another links the two, and a group of
+   linked locals takes one type. *)
+type lit_con = Up | Down | Hint
+
+type lit_session = {
+  (* The type each literal local is checked at, by its initialiser's node. *)
+  mutable decided : (Ast.expr * Types.t) list;
+  (* On during the recording check and off for the real one. *)
+  mutable recording : bool;
+  mutable cons : (Ast.expr * (lit_con * Types.t * Loc.t)) list;
+  mutable links : (Ast.expr * Ast.expr) list;
+  (* Each literal local the recording check bound, with its name. *)
+  mutable seen : (Ast.expr * string) list;
+}
+
+(* Nonzero while any recording check runs: the refusal memos ([arm_failed],
+   [if_failed], [truthy_failed]) are not written then, since a refusal made
+   at a guessed type must not be replayed at the decided one. *)
+let lit_recording = ref 0
+
+(* Set around the one check of an operator's operand that is a literal local,
+   so its use is recorded as a [Hint] and not an [Up]. *)
+let lit_hint = ref false
+
 (* Per-function state. Slots are never reused, so [slots] is also the frame
    size — the interpreter allocates one array of this length per call. *)
 type ctx = {
   env           : env;
   ret           : Types.t;
+  (* The literal-inference session of this function, while one is open. *)
+  mutable lits  : lit_session option;
   mutable slots : int;
   (* The type of each slot, newest first. A backend needs it to size the
      frame — nothing else records it, since the IR refers to slots by index. *)
@@ -910,7 +956,7 @@ let render_ctx ctx (emit : Render.emitter) : Render.ctx =
    variables properly means emitting a [!DILexicalBlock] per [Let] and moving
    the [llvm.dbg.declare]s out of the entry block to the binding sites, which
    needs block structure this IR does not carry. *)
-let bind ctx ?what name bty ~assignable =
+let bind ctx ?what ?lit name bty ~assignable =
   let taken n = List.exists (fun s -> s = Some n) ctx.slot_names in
   let name' =
     if not (taken name) then name
@@ -924,7 +970,7 @@ let bind ctx ?what name bty ~assignable =
   let slot = fresh_slot ~name:name' ctx bty in
   (* [ctx.scope] keeps the *source* name: the suffix is a debug-info artifact
      and resolving [v] must still find the innermost binding. *)
-  ctx.scope <- (name, { slot; bty; assignable; bwhat = what }) :: ctx.scope;
+  ctx.scope <- (name, { slot; bty; assignable; bwhat = what; blit = lit }) :: ctx.scope;
   slot
 
 let lookup ctx name = List.assoc_opt name ctx.scope
@@ -956,7 +1002,7 @@ let rec capture ctx loc name =
      a [let] inside the body restores what it displaced, and the copy's
      binding goes with it. One field, not two: the environment is keyed by the
      source name. *)
-  | Some (outer, slot) -> Some { slot; bty = outer.bty; assignable = false; bwhat = None }
+  | Some (outer, slot) -> Some { slot; bty = outer.bty; assignable = false; bwhat = None; blit = None }
   | None ->
     let from_parent () =
       (* Not a local of the body directly around this one, so ask whether that
@@ -977,7 +1023,7 @@ let rec capture ctx loc name =
     | Some _, Some (outer : binding) ->
       let slot = bind ctx name outer.bty ~assignable:false in
       ctx.caught <- ctx.caught @ [ (name, (outer, slot)) ];
-      Some { slot; bty = outer.bty; assignable = false; bwhat = None }
+      Some { slot; bty = outer.bty; assignable = false; bwhat = None; blit = None }
     | _ -> None
 
 (* The one thing [capture] does not answer for. A captured name is a copy, so
@@ -997,7 +1043,7 @@ and peek_outer ctx name =
   if ctx.outer_what = None then None
   else
     match List.assoc_opt name ctx.caught with
-    | Some ((b : binding), slot) -> Some { slot; bty = b.bty; assignable = false; bwhat = None }
+    | Some ((b : binding), slot) -> Some { slot; bty = b.bty; assignable = false; bwhat = None; blit = None }
     | None ->
       match List.assoc_opt name ctx.outer with
       | Some b -> Some b
@@ -3585,6 +3631,114 @@ let restart_sig tys =
 let dyn_i64 = Types.Int Types.I64
 let dyn_f64 = Types.Float Types.F64
 
+(* PROTOTYPE switch for measuring the literal rules; removed before merge. *)
+let lit_mode = try Sys.getenv "FLAN_LIT" with Not_found -> ""
+let lit_has m = List.mem m (String.split_on_char ',' lit_mode)
+let float_default () = if lit_has "f32" then Types.F32 else Types.F64
+
+(* ── Literal locals ([lit_session]) ────────────────────────────────── *)
+
+(* The literal a [let] or [loop] initialiser is, when its type is to be read
+   off the uses: a number or a character, negated or not. A bool has one type
+   and a wide literal one (u64), so neither has anything to infer. *)
+let lit_kind (e : Ast.expr) =
+  match e.Ast.e with
+  | Ast.Int _ -> Some `Int
+  | Ast.Byte _ -> Some `Char
+  | Ast.Float _ -> Some `Float
+  | Ast.Call ({ Ast.e = Ast.Var "-"; _ }, [ { Ast.e = Ast.Int _; _ } ]) -> Some `Int
+  | Ast.Call ({ Ast.e = Ast.Var "-"; _ }, [ { Ast.e = Ast.Float _; _ } ]) -> Some `Float
+  | _ -> None
+
+(* What the literal is with no use to say otherwise. *)
+let lit_default = function
+  | `Int -> Types.Int Types.I32
+  | `Char -> Types.Int Types.U8
+  | `Float -> Types.Float (float_default ())
+
+(* The types a use can give it: any number for an integer or a character,
+   since an untyped integer constant is usable where a float is wanted, and
+   only a float for a float. A type variable is admitted and left to
+   [int_literal] to judge against its bound. Anything else — dyn, a struct —
+   says nothing about the literal's type; the local keeps its guess and the
+   use is checked as it always was. *)
+let lit_admits kind (t : Types.t) =
+  match kind, t with
+  | (`Int | `Char), (Types.Int _ | Types.Float _ | Types.Var _) -> true
+  | `Float, (Types.Float _ | Types.Var _) -> true
+  | _ -> false
+
+let lit_rounds = 3
+
+(* Every literal local the recording check bound, with the type the uses
+   decide for it, and the first pair of uses that no one type satisfies. *)
+let lit_solve (s : lit_session) =
+  let keys =
+    List.fold_left
+      (fun acc (k, n) -> if List.exists (fun (k', _) -> k' == k) acc then acc else (k, n) :: acc)
+      [] s.seen
+  in
+  (* The linked group of [k]: a [set] of one literal local into another. *)
+  let group k =
+    let rec go seen = function
+      | [] -> seen
+      | k :: rest when List.memq k seen -> go seen rest
+      | k :: rest ->
+        let next =
+          List.filter_map
+            (fun (a, b) -> if a == k then Some b else if b == k then Some a else None)
+            s.links
+        in
+        go (k :: seen) (next @ rest)
+    in
+    go [] [ k ]
+  in
+  let widens a b = Types.equal a b || Types.widens_to ~from:a ~into:b in
+  List.map
+    (fun (k, name) ->
+       let members = List.filter (fun m -> List.exists (fun (k', _) -> k' == m) keys) (group k) in
+       let kind =
+         if List.exists (fun m -> lit_kind m = Some `Float) members then `Float
+         else Option.value (lit_kind k) ~default:`Int
+       in
+       let cons =
+         List.filter_map
+           (fun (k', c) -> if List.memq k' members then Some c else None)
+           s.cons
+         |> List.filter (fun (_, t, _) -> lit_admits kind t)
+         |> List.rev
+       in
+       let pick c = List.filter_map (fun (c', t, l) -> if c' = c then Some (t, l) else None) cons in
+       let ups = pick Up and downs = pick Down and hints = pick Hint in
+       let res =
+         match ups with
+         | (u0, l0) :: _ ->
+           (match List.find_opt (fun (u, _) -> List.for_all (fun (u', _) -> widens u u') ups) ups with
+            | None ->
+              let (u1, l1) =
+                List.find (fun (u, _) -> not (widens u u0 || widens u0 u)) ups
+              in
+              Error ((u0, l0), (u1, l1))
+            | Some (c, lc) ->
+              (match List.find_opt (fun (d, _) -> not (widens d c)) downs with
+               | None -> Ok c
+               | Some (d, ld) -> Error ((c, lc), (d, ld))))
+         | [] ->
+           (match downs @ hints with
+            | [] -> Ok (lit_default kind)
+            | (t0, l0) :: rest ->
+              let rec fold (t, l) = function
+                | [] -> Ok t
+                | (t', l') :: rest ->
+                  (match Types.join t t' with
+                   | Some j -> fold ((j, if Types.equal j t then l else l')) rest
+                   | None -> Error ((t, l), (t', l')))
+              in
+              fold (t0, l0) rest)
+       in
+       (k, name, res))
+    keys
+
 (* Converting to whatever width the other side of the boundary wants, with a
    [Cast] and not a silent reinterpretation. The name is for the direction it
    was written for: runtime/flan_dyn.h boxes integers as [i64] and floats as
@@ -4763,7 +4917,7 @@ let with_recovery env ~on f =
   end
 
 let invented_ctx env ret =
-  { env; ret; slots = 0; slot_tys = []; slot_names = []; scope = [];
+  { env; ret; lits = None; slots = 0; slot_tys = []; slot_names = []; scope = [];
     defers = []; defer_slot = None; outer = []; outer_what = None; caught = []; place_ok = false; envslot = None; parent = None; in_frames = None; loops = []; tail = false;
     in_defer = false; defer_ok = false; defer_block = "a nested form";
     owner = "<none>" }
@@ -5711,9 +5865,10 @@ and check_value ctx ?want (e : Ast.expr) : Tast.expr =
       | Some other when other <> Types.Never ->
         Loc.failk literal_at_want loc "expected %s, found the float literal %g"
           (tyname loc other) x
-      | _ -> Types.F64
+      | _ -> float_default ()
     in
     mk loc (Types.Float k) (Tast.Float (x, k))
+  | Ast.Str s when want = None && lit_has "dyn" -> box loc (mk loc Types.String (Tast.Str s))
   | Ast.Str s -> expect ctx loc ~want (mk loc Types.String (Tast.Str s))
   | Ast.Kw k ->
     (* Two keywords in one spelling, told apart by the expectation. Where an
@@ -5962,6 +6117,11 @@ and check_value ctx ?want (e : Ast.expr) : Tast.expr =
     let v = check ctx ~want:Types.Dyn v in
     expect ctx loc ~want
       (rt loc Types.Unit "flan_dyn_slot_set" [ target; k; v; here loc ])
+  | Ast.Set ((Ast.Pvar n as p), v) when lit_recorded ctx n <> None ->
+    let key = Option.get (lit_recorded ctx n) in
+    let p, pty = check_place ctx loc p in
+    let v = lit_down ctx key pty v in
+    expect ctx loc ~want (mk loc Types.Unit (Tast.Set (p, v)))
   | Ast.Set (p, v) ->
     let p, pty = check_place ctx loc p in
     let v = check ctx ~want:pty v in
@@ -5984,6 +6144,8 @@ and check_value ctx ?want (e : Ast.expr) : Tast.expr =
      fixed-array literal they always were. *)
   | Ast.Arr items when want = Some Types.Dyn ->
     dyn_vec ctx loc (map_lr (fun x -> check ctx ~want:Types.Dyn x) items)
+  | Ast.Arr items when want = None && lit_has "dyn" ->
+    dyn_vec ctx loc (map_lr (fun x -> check ctx ~want:Types.Dyn x) items)
   | Ast.Arr items -> check_arr ctx ~want loc items
   (* (array 4 rl/Vector2). Parse already assembled the whole array type, so
      there is nothing to infer: resolve it and hand back its all-bytes-zero
@@ -6338,6 +6500,16 @@ and var ctx ?(qualified = false) loc ~want name =
        defn to pass that" builtin_prefix name name builtin_prefix name
   | _ ->
     match lookup ctx name with
+    (* A literal local while its uses are being recorded: the use is noted
+       and read at the type it asks for, so the recording check goes on past
+       a use its guess would have refused. That check is thrown away. *)
+    | Some ({ blit = Some key; _ } as b)
+      when (match ctx.lits, want with
+            | Some s, Some t -> s.recording && lit_admits (Option.value (lit_kind key) ~default:`Int) t
+            | _ -> false) ->
+      let s = Option.get ctx.lits and t = Option.get want in
+      s.cons <- (key, ((if !lit_hint then Hint else Up), t, loc)) :: s.cons;
+      mk loc t (Tast.Local b.slot)
     | Some b ->
       expect ctx loc ~want (mk loc b.bty (Tast.Local b.slot))
     (* A local of the enclosing function, in a body that was lifted out of it:
@@ -7050,12 +7222,140 @@ and defer_counter_zero slot loc =
 (* [defer_ok] says whether *this* let has the function's extent. If it does, so
    does every form in its body, including a nested let — which is why the flag
    is handed to the body rather than consumed here. *)
+(* The literal local [n] names, while its uses are being recorded. *)
+and lit_recorded ctx n =
+  match ctx.lits, lookup ctx n with
+  | Some s, Some { blit = Some key; _ } when s.recording -> Some key
+  | _ -> None
+
+(* [v] stored into the literal local [key] (a [set] or a [recur]), while
+   recording: checked on its own terms, so its type is what it brings rather
+   than the guess. Another literal local links the two; a float literal says
+   only that it is a float. *)
+and lit_down ctx key pty (v : Ast.expr) =
+  let s = Option.get ctx.lits in
+  let other =
+    match v.Ast.e with
+    | Ast.Var m -> (match lookup ctx m with Some { blit = Some k; _ } -> Some k | _ -> None)
+    | _ -> None
+  in
+  match other with
+  | Some k -> s.links <- (key, k) :: s.links; check ctx v
+  | None ->
+    match lit_kind v with
+    | Some `Float ->
+      s.cons <- (key, (Hint, Types.Float (float_default ()), v.Ast.loc)) :: s.cons;
+      check ctx v
+    (* An integer literal fits wherever its value does; one past i32 says
+       the local is at least an i64. *)
+    | Some _ ->
+      (match v.Ast.e with
+       | Ast.Int n when Int64.compare n (Int64.of_int32 Int32.max_int) > 0
+                        || Int64.compare n (Int64.of_int32 Int32.min_int) < 0 ->
+         s.cons <- (key, (Hint, Types.Int Types.I64, v.Ast.loc)) :: s.cons;
+         check ctx ~want:(Types.Int Types.I64) v
+       | _ -> check ctx ~want:pty v)
+    | None ->
+      (match trial ctx (fun () -> check ctx v) with
+       | Ok e ->
+         s.cons <- (key, (Down, e.Tast.ty, v.Ast.loc)) :: s.cons;
+         e
+       | Error _ -> check ctx ~want:pty v)
+
+(* The type a literal initialiser is checked at while a session is open —
+   its current guess, or the decision — noting it as seen while recording.
+   [None] for anything that is not a literal, or with no session open. *)
+and lit_local ctx name (e : Ast.expr) =
+  match ctx.lits, lit_kind e with
+  | Some s, Some kind ->
+    if s.recording then s.seen <- (e, name) :: s.seen;
+    Some (match List.assq_opt e s.decided with Some t -> t | None -> lit_default kind)
+  | _ -> None
+
+(* [run] is a [let] or a [loop] with some of [inits] literals, and the
+   outermost such form of this function: the session opens here. See
+   [lit_session]. *)
+and with_lits : 'a. ctx -> Loc.t -> Ast.expr list -> (unit -> 'a) -> 'a =
+  fun ctx loc inits run ->
+  if ctx.lits <> None || not (List.exists (fun e -> lit_kind e <> None) inits)
+  then run ()
+  else begin
+    let s = { decided = []; recording = false; cons = []; links = []; seen = [] } in
+    ctx.lits <- Some s;
+    Fun.protect ~finally:(fun () -> ctx.lits <- None) @@ fun () ->
+    let undo = Loc.diag ~kind:"check/lit-undo" loc "undone" in
+    let rec round n =
+      s.cons <- []; s.links <- []; s.seen <- [];
+      s.recording <- true;
+      incr lit_recording;
+      Fun.protect
+        ~finally:(fun () -> decr lit_recording; s.recording <- false)
+        (fun () -> ignore (trial ctx (fun () -> ignore (run ()); raise (Loc.Error undo))));
+      let solved = lit_solve s in
+      let guess k =
+        match List.assq_opt k s.decided with
+        | Some t -> t
+        | None -> lit_default (Option.value (lit_kind k) ~default:`Int)
+      in
+      let decided =
+        List.map (fun (k, _, r) -> (k, match r with Ok t -> t | Error _ -> guess k)) solved
+      in
+      let moved = List.exists (fun (k, t) -> not (Types.equal t (guess k))) decided in
+      s.decided <- decided;
+      if moved && n < lit_rounds then round (n + 1)
+      else
+        match List.find_opt (fun (_, _, r) -> Result.is_error r) solved with
+        | Some (k, name, Error ((t1, l1), (t2, l2))) -> lit_conflict k name t1 l1 t2 l2
+        | _ -> ()
+    in
+    round 1;
+    if lit_has "log" then
+      List.iter
+        (fun (k, t) ->
+           let d = lit_default (Option.value (lit_kind k) ~default:`Int) in
+           if not (Types.equal t d) then
+             Printf.eprintf "LITINF %s:%d:%d %s -> %s\n" k.Ast.loc.Loc.file
+               k.Ast.loc.Loc.line k.Ast.loc.Loc.col (tyname loc d) (tyname loc t))
+        s.decided;
+    run ()
+  end
+
+(* Two uses of a literal local that no one type satisfies. *)
+and lit_conflict (k : Ast.expr) name t1 l1 t2 l2 =
+  let lit =
+    match k.Ast.e with
+    | Ast.Int n -> Int64.to_string n
+    | Ast.Float x -> Printf.sprintf "%g" x
+    | Ast.Byte b -> Printf.sprintf "\\%c" (Char.chr b)
+    | Ast.Call (_, [ { Ast.e = Ast.Int n; _ } ]) -> Int64.to_string (Int64.neg n)
+    | Ast.Call (_, [ { Ast.e = Ast.Float x; _ } ]) -> Printf.sprintf "%g" (-.x)
+    | _ -> "..."
+  in
+  let lit = if String.length lit > 0 && lit.[0] <> '-' && not (String.contains lit '.') && lit_kind k = Some `Float then lit ^ ".0" else lit in
+  let fix =
+    if fln_source k.Ast.loc then Printf.sprintf "let %s: %s = %s" name (tyname l1 t1) lit
+    else Printf.sprintf "(%s %s)" (tyname l1 t1) lit
+  in
+  Loc.failk "check/literal-uses" k.Ast.loc
+    ~notes:[ Loc.note l1 (Printf.sprintf "%s is used as %s here" name (tyname l1 t1));
+             Loc.note l2 (Printf.sprintf "and as %s here" (tyname l2 t2)) ]
+    "%s is used as %s and as %s, and %s can have only one type. Write the \
+     one it should have: %s"
+    name (tyname l1 t1) (tyname l2 t2) lit fix
+
 and check_let ctx ?(tail = false) ?want ?(defer_ok = false) loc bs body =
+  with_lits ctx loc
+    (List.filter_map
+       (fun (b : Ast.binding) -> if b.Ast.bty = None then Some b.Ast.bval else None)
+       bs)
+  @@ fun () ->
   scoped ctx (fun () ->
     let bs =
       map_lr
         (fun (b : Ast.binding) ->
            let want = Option.map (resolve ctx.env) b.Ast.bty in
+           let lit = if b.Ast.bty = None then lit_local ctx b.Ast.bname b.Ast.bval else None in
+           let want = match lit with Some t -> Some t | None -> want in
            let v = check ctx ?want b.Ast.bval in
            (match v.Tast.ty with
             (* A refused initialiser, already reported: the name is bound to
@@ -7066,7 +7366,10 @@ and check_let ctx ?(tail = false) ?want ?(defer_ok = false) loc bs body =
                 b.Ast.bname (tyname loc v.Tast.ty)
             | _ -> ());
            (* Locals are assignable places; parameters are not. *)
-           let slot = bind ctx b.Ast.bname v.Tast.ty ~assignable:true in
+           let slot =
+             bind ctx b.Ast.bname v.Tast.ty ~assignable:true
+               ?lit:(Option.map (fun _ -> b.Ast.bval) lit)
+           in
            (slot, v))
         bs
     in
@@ -7281,6 +7584,7 @@ and check_dotimes ctx ~want loc label name (b : Ast.bounds) body =
    below and every [continue] a [recur] mints count from the same stack [emit]
    indexes. *)
 and check_loop ctx ?want loc bs body =
+  with_lits ctx loc (List.map snd bs) @@ fun () ->
   scoped ctx (fun () ->
     (* Each initial value is evaluated once, before the loop, exactly as a
        [let]'s is and as [dotimes]'s bound is — and bound before the next is
@@ -7288,8 +7592,9 @@ and check_loop ctx ?want loc bs body =
        name. *)
     let binds =
       map_lr
-        (fun (n, v) ->
+        (fun (n, v0) ->
-           let v = check ctx v in
+           let lit = lit_local ctx n v0 in
+           let v = check ctx ?want:lit v0 in
            (match v.Tast.ty with
             (* A refused initialiser, already reported: the name is bound to
                the poison so that what follows is still checked. *)
@@ -7298,7 +7603,8 @@ and check_loop ctx ?want loc bs body =
               fail v.Tast.loc "%s would be bound to %s, which is not a value" n
                 (tyname loc v.Tast.ty)
             | _ -> ());
-           (bind ctx n v.Tast.ty ~assignable:true, v))
+           (bind ctx n v.Tast.ty ~assignable:true
+              ?lit:(Option.map (fun _ -> v0) lit), v))
         bs
     in
     let names = List.map (fun (slot, v) -> (slot, v.Tast.ty)) binds in
@@ -7371,7 +7677,18 @@ and check_recur ctx ~tail loc args =
   if want <> got then
     fail loc "this loop binds %d name%s and this recur passes %d" want
       (if want = 1 then "" else "s") got;
-  let vals = List.map2 (fun a (_, ty) -> check ctx ~want:ty a) args names in
+  let vals =
+    List.map2
+      (fun a (slot, ty) ->
+         match
+           List.find_opt (fun (_, (b : binding)) -> b.slot = slot) ctx.scope
+         with
+         | Some (_, { blit = Some key; _ })
+           when (match ctx.lits with Some s -> s.recording | None -> false) ->
+           lit_down ctx key ty a
+         | _ -> check ctx ~want:ty a)
+      args names
+  in
   (* Every name is rebound at once. The new values go into temporaries first,
      so that (recur y x) swaps rather than writing y over x and then reading it
      back — the same reason Clojure's recur is simultaneous. *)
@@ -7486,7 +7803,7 @@ and check_truthy ctx c =
       (fun () ->
          try check_truthy_once ctx c
          with Loc.Error d as ex ->
-           truthy_failed := (c, scope, ctx.ret, d) :: !truthy_failed;
+           if !lit_recording = 0 then truthy_failed := (c, scope, ctx.ret, d) :: !truthy_failed;
            raise ex)
 
 and check_truthy_once ctx c =
@@ -7564,7 +7881,7 @@ and check_if ctx ?(tail = false) ?want loc c t e =
       (fun () ->
          try check_if_once ctx ~tail ?want loc c t e
          with Loc.Error d as ex ->
-           Hashtbl.add if_failed c.Ast.loc (c, (scope, ctx.ret), want, d);
+           if !lit_recording = 0 then Hashtbl.add if_failed c.Ast.loc (c, (scope, ctx.ret), want, d);
            raise ex)
 
 and check_if_once ctx ~tail ?want loc c t e =
@@ -7653,7 +7970,7 @@ and check_if_once ctx ~tail ?want loc c t e =
             with
             | Ok v -> Ok v
             | Error d ->
-              Hashtbl.add arm_failed e.Ast.loc (e, key, t.Tast.ty, d);
+              if !lit_recording = 0 then Hashtbl.add arm_failed e.Ast.loc (e, key, t.Tast.ty, d);
               Error d
         in
         let meet v =
@@ -7843,7 +8160,7 @@ and generic_ctor ctx ~want loc name given =
     (* A literal's own type, the one it has with nothing expected of it. *)
     let literal_type (a : Ast.expr) =
       match a.Ast.e with
-      | Ast.Float _ -> Types.Float Types.F64
+      | Ast.Float _ -> Types.Float (float_default ())
       | Ast.UInt _ -> Types.Int Types.U64
       | Ast.Byte _ -> Types.Int Types.U8
       | _ -> Types.Int Types.I32
@@ -9241,7 +9558,7 @@ and check_match ctx ?(tail = false) ?want loc scrutinee arms =
                    (match trial ctx (at (Some w)) with
                     | Ok b -> Ok b
                     | Error d ->
-                      Hashtbl.add arm_failed head.Ast.loc
+                      if !lit_recording = 0 then Hashtbl.add arm_failed head.Ast.loc
                         (head, (ctx.scope, ctx.ret), w, d);
                       Error d)
                in
@@ -14144,7 +14461,7 @@ and trial ctx f =
      Only [Loc.Error] is caught. A timeout or a stack overflow is not a
      refusal to reconsider, and silently continuing past one would turn a
      resource failure into a wrong answer. *)
-  let[@warning "+9"] { env = _; ret = _; slots; slot_tys; slot_names; scope;
+  let[@warning "+9"] { env = _; ret = _; lits = _; slots; slot_tys; slot_names; scope;
                        defers; defer_slot; defer_ok; defer_block; outer = _;
                        outer_what; caught; place_ok; envslot; parent = _;
                        in_frames; loops; tail; in_defer;
@@ -14207,7 +14524,7 @@ and trial_at ctx (y : Ast.expr) (w : Types.t) =
     (match trial ctx (fun () -> check ctx ~want:w y) with
      | Ok b -> Ok b
      | Error d ->
-       Hashtbl.add arm_failed y.Ast.loc (y, (ctx.scope, ctx.ret), w, d);
+       if !lit_recording = 0 then Hashtbl.add arm_failed y.Ast.loc (y, (ctx.scope, ctx.ret), w, d);
        Error d)
 
 and binary ctx ?(dyn_ok = false) ?(join = true) name loc ~want args =
@@ -14227,7 +14544,35 @@ and binary ctx ?(dyn_ok = false) ?(join = true) name loc ~want args =
     let needs_want (f : Ast.expr) =
       is_literal f || (match f.Ast.e with Ast.Kw _ -> true | _ -> false)
     in
-    if y_decides then begin
+    (* While a literal local's uses are recorded, the operand beside it
+       decides and the local is recorded as meeting it ([lit_session]). *)
+    let lv (f : Ast.expr) =
+      match f.Ast.e with Ast.Var n -> lit_recorded ctx n <> None | _ -> false
+    in
+    let hinted f =
+      lit_hint := true;
+      Fun.protect ~finally:(fun () -> lit_hint := false) f
+    in
+    let float_lit (f : Ast.expr) = lit_kind f = Some `Float in
+    if lv x && float_lit y then begin
+      let a = hinted (fun () -> check ctx ~want:(Types.Float (float_default ())) x) in
+      a, check ctx ~want:a.Tast.ty y
+    end
+    else if lv y && float_lit x then begin
+      let b = hinted (fun () -> check ctx ~want:(Types.Float (float_default ())) y) in
+      check ctx ~want:b.Tast.ty x, b
+    end
+    else if lv x && not (lv y) && not (needs_want y) then begin
+      let b = check ctx ?want y in
+      let a = hinted (fun () -> check ctx ~want:b.Tast.ty x) in
+      a, b
+    end
+    else if lv y && not (lv x) && not (needs_want x) then begin
+      let a = check ctx ?want x in
+      let b = hinted (fun () -> check ctx ~want:a.Tast.ty y) in
+      a, b
+    end
+    else if y_decides then begin
       let b = check ctx ?want y in
       let a = check ctx ~want:b.Tast.ty x in
       a, b

test/programs/literal-locals.flan

@@ -0,0 +1,59 @@
+;;;; A number literal bound by let or loop takes its type from its uses in
+;;;; the function. Each line's expected output is beside it.
+
+;; A set of an i64 sum makes the accumulator an i64.
+(defn total [xs [i64]] i64
+  (let [t 0]
+    (dotimes [i (length xs)]
+      (set t (+ t (at xs i))))
+    t))
+
+;; The operand beside it: an f64 accumulator from a float literal.
+(defn mean [xs [f64]] f64
+  (let [s 0.0]
+    (dotimes [i (length xs)]
+      (set s (+ s (at xs i))))
+    (/ s (f64 (length xs)))))
+
+;; A counter compared with an i64 bound counts past i32.
+(defn count-to [n i64] i64
+  (let [i 0]
+    (while (< i n)
+      (set i (+ i 1000000000)))
+    i))
+
+;; A set of one literal local into another links them: b holds a value past
+;; i32, so a is an i64 too.
+(defn linked [] i64
+  (let [a 0 b 0]
+    (set b 3000000000)
+    (set a b)
+    a))
+
+;; recur rebinds a loop's names the way set does.
+(defn sum-to [n i64] i64
+  (loop [i 0 acc 0]
+    (if (< i n) (recur (+ i 1) (+ acc 1000000000)) acc)))
+
+;; Inside a generic body the literal takes the type variable.
+(defn sum-of [xs [$t]] $t {:where (numeric? $t)}
+  (let [acc 0]
+    (dotimes [i (length xs)]
+      (set acc (+ acc (at xs i))))
+    acc))
+
+(defn main [] i32
+  (let [xs (the [3 i64] [3000000000 4 5])
+        fs (the [2 f64] [0.5 0.25])
+        gs (the [2 u8] [200 50])]
+    (println (total (slice xs 0 3)))               ; 3000000009
+    (println (mean (slice fs 0 2)))                ; 0.375
+    (println (count-to 5000000000))                ; 5000000000
+    (println (linked))                             ; 3000000000
+    (println (sum-to 3))                           ; 3000000000
+    (println (sum-of (slice xs 0 3)))              ; 3000000009
+    (println (sum-of (slice fs 0 2))))             ; 0.75
+  ;; Nothing says otherwise: an i32 and an f64.
+  (let [n 7 f 1.5]
+    (println n f))                                 ; 7 1.5
+  0)

test/test_acceptance.ml

@@ -389,6 +389,14 @@ let () =
     outputs "value semantics" "programs/values.flan" values_out;
     outputs "machine surface" "programs/machine.flan" machine_out;
     outputs "unit main exits 0" "programs/unit-main.flan" "ok\n";
+    let literal_locals_out =
+      "3000000009\n0.375\n5000000000\n3000000000\n3000000000\n3000000009\n\
+       0.75\n7 1.5\n"
+    in
+    outputs "literal locals take their uses' type" "programs/literal-locals.flan"
+      literal_locals_out;
+    outputs ~x86:true "literal locals take their uses' type, --x86"
+      "programs/literal-locals.flan" literal_locals_out;
     (* Comparisons over three operands and more. The lines that carry the
        whole claim are the tag transcripts: [abc -> false] is a chain whose
        *first* link already decided the answer and whose middle operand —

test/test_flan.ml

@@ -1086,6 +1086,21 @@ let () =
      two [infers] above still hold — and this is the position that had no way
      to say it. *)
   infers "array constructor"     "(array 4 f32)" "[4 f32]";
+  (* A literal bound by a let takes its type from its uses in the function,
+     and two uses no one type satisfies are refused with the annotation. *)
+  accepts "a literal local takes the type set into it"
+    "(defn f [x i64] i64 (let [t 0] (set t (+ t x)) t))";
+  accepts "a literal local takes an operand's type"
+    "(defn f [x f64] f64 (let [s 0.0] (set s (+ s x)) s))";
+  accepts "recur rebinds a literal local at the type it brings"
+    "(defn f [n i64] i64 (loop [i 0 acc 0] (if (< i n) (recur (+ i 1) (+ acc n)) acc)))";
+  accepts "a set links two literal locals"
+    "(defn f [] i64 (let [a 0 b 0] (set b 3000000000) (set a b) a))";
+  rejects_check "two uses of a literal local disagree"
+    ~needle:"x is used as u32 and as i32, and 0 can have only one type. \
+             Write the one it should have: (u32 0)"
+    "(defn u [x u32] u32 x) (defn i [x i32] i32 x) \
+     (defn f [] i32 (let [x 0] (u x) (i x)) 0)";
   infers "array of a struct"     "(array 2 i32)" "[2 i32]";
   infers "array of an array"     "(array 2 [3 u8])" "[2 [3 u8]]";
   (* (array-fill [r c] v): the same type at any rank, with the element type