Use type annotations from arguments in let rec (#12315)

Changes

@@ -12,6 +12,9 @@ Working version
   (Jeremy Yallop, review by Nicolás Ojeda Bär and Gabriel Scherer,
   suggestion by Rodolphe Lepigre and John Whitington)
 
+- #12315: Use type annotations from arguments in let rec
+  (Stephen Dolan, review by Gabriel Scherer)
+
 ### Runtime system:
 
 - #10111: Increase the detail of location information for debugging events to

testsuite/tests/typing-misc/let_rec_approx.ml

@@ -0,0 +1,38 @@
+(* TEST
+ expect;
+*)
+
+module M = struct type t = A | B end
+let rec f () = g A
+and g (x : M.t) = f ()
+[%%expect{|
+module M : sig type t = A | B end
+val f : unit -> 'a = <fun>
+val g : M.t -> 'a = <fun>
+|}]
+
+let rec f () = g 42
+and g (x : string) = f ()
+[%%expect{|
+Line 1, characters 17-19:
+1 | let rec f () = g 42
+                     ^^
+Error: This expression has type "int" but an expression was expected of type
+         "string"
+|}]
+
+let rec opt_error ?(opt : string) () = f ?opt ()
+[%%expect{|
+Line 1, characters 20-32:
+1 | let rec opt_error ?(opt : string) () = f ?opt ()
+                        ^^^^^^^^^^^^
+Error: This pattern matches values of type "string"
+       but a pattern was expected which matches values of type "'a option"
+|}]
+
+let rec opt_ok_f () = opt_ok_g ~foo:A ()
+and opt_ok_g ?(foo : M.t option) () = opt_ok_f ()
+[%%expect{|
+val opt_ok_f : unit -> 'a = <fun>
+val opt_ok_g : ?foo:M.t -> unit -> 'a = <fun>
+|}]

typing/typecore.ml

@@ -2540,11 +2540,18 @@ let rec approx_type env sty =
       approx_type env sty
   | _ -> newvar ()
 
+let type_pattern_approx env spat =
+  match spat.ppat_desc with
+  | Ppat_constraint (_, sty) -> approx_type env sty
+  | _ -> newvar ()
+
 let rec type_approx env sexp =
   match sexp.pexp_desc with
     Pexp_let (_, _, e) -> type_approx env e
-  | Pexp_fun (p, _, _, e) ->
-      let ty = if is_optional p then type_option (newvar ()) else newvar () in
+  | Pexp_fun (p, _, spat, e) ->
+      let ty = type_pattern_approx env spat in
+      if is_optional p then
+        unify_pat_types spat.ppat_loc env ty (type_option (newvar ()));
       newty (Tarrow(p, ty, type_approx env e, commu_ok))
   | Pexp_function ({pc_rhs=e}::_) ->
       newty (Tarrow(Nolabel, newvar (), type_approx env e, commu_ok))