Fix parameter type / return type ambiguity error for unit case

CHANGELOG.md

@@ -23,6 +23,7 @@
 #### :bug: Bug fix
 
 - Prevent lockfile creation when project folder is missing. https://github.com/rescript-lang/rescript/pull/7927
+- Fix parameter type / return type ambiguity error for unit case. https://github.com/rescript-lang/rescript/pull/7930
 
 #### :memo: Documentation
 

compiler/syntax/src/res_core.ml

@@ -2554,54 +2554,62 @@ and over_parse_constrained_or_coerced_or_arrow_expression p expr =
     | EqualGreater ->
       Parser.next p;
       let body = parse_expr p in
-      let pat =
+      let pat, expr_is_unit =
         match expr.pexp_desc with
         | Pexp_ident longident ->
-          Ast_helper.Pat.var ~loc:expr.pexp_loc
-            (Location.mkloc
-               (Longident.flatten longident.txt |> String.concat ".")
-               longident.loc)
+          ( Ast_helper.Pat.var ~loc:expr.pexp_loc
+              (Location.mkloc
+                 (Longident.flatten longident.txt |> String.concat ".")
+                 longident.loc),
+            false )
+        | Pexp_construct (({txt = Longident.Lident "()"} as lid), None) ->
+          (Ast_helper.Pat.construct ~loc:expr.pexp_loc lid None, true)
         (* TODO: can we convert more expressions to patterns?*)
         | _ ->
-          Ast_helper.Pat.var ~loc:expr.pexp_loc
-            (Location.mkloc "pattern" expr.pexp_loc)
+          ( Ast_helper.Pat.var ~loc:expr.pexp_loc
+              (Location.mkloc "pattern" expr.pexp_loc),
+            false )
       in
       let arrow1 =
         Ast_helper.Exp.fun_
           ~loc:(mk_loc expr.pexp_loc.loc_start body.pexp_loc.loc_end)
           ~arity:None Asttypes.Nolabel None pat
           (Ast_helper.Exp.constraint_ body typ)
       in
-      let arrow2 =
-        Ast_helper.Exp.fun_
-          ~loc:(mk_loc expr.pexp_loc.loc_start body.pexp_loc.loc_end)
-          ~arity:None Asttypes.Nolabel None
-          (Ast_helper.Pat.constraint_ pat typ)
-          body
-      in
-      let msg =
-        Doc.breakable_group ~force_break:true
-          (Doc.concat
-             [
-               Doc.text
-                 "Did you mean to annotate the parameter type or the return \
-                  type?";
-               Doc.indent
-                 (Doc.concat
-                    [
-                      Doc.line;
-                      Doc.text "1) ";
-                      ResPrinter.print_expression arrow1 CommentTable.empty;
-                      Doc.line;
-                      Doc.text "2) ";
-                      ResPrinter.print_expression arrow2 CommentTable.empty;
-                    ]);
-             ])
-        |> Doc.to_string ~width:80
-      in
-      Parser.err ~start_pos:expr.pexp_loc.loc_start
-        ~end_pos:body.pexp_loc.loc_end p (Diagnostics.message msg);
-      arrow1
+      (* When the "expr" was `()`, the colon must apply to the return type, so
+         skip the ambiguity diagnostic and keep the parameter as unit. *)
+      if expr_is_unit then arrow1
+      else
+        let arrow2 =
+          Ast_helper.Exp.fun_
+            ~loc:(mk_loc expr.pexp_loc.loc_start body.pexp_loc.loc_end)
+            ~arity:None Asttypes.Nolabel None
+            (Ast_helper.Pat.constraint_ pat typ)
+            body
+        in
+        let msg =
+          Doc.breakable_group ~force_break:true
+            (Doc.concat
+               [
+                 Doc.text
+                   "Did you mean to annotate the parameter type or the return \
+                    type?";
+                 Doc.indent
+                   (Doc.concat
+                      [
+                        Doc.line;
+                        Doc.text "1) ";
+                        ResPrinter.print_expression arrow1 CommentTable.empty;
+                        Doc.line;
+                        Doc.text "2) ";
+                        ResPrinter.print_expression arrow2 CommentTable.empty;
+                      ]);
+               ])
+          |> Doc.to_string ~width:80
+        in
+        Parser.err ~start_pos:expr.pexp_loc.loc_start
+          ~end_pos:body.pexp_loc.loc_end p (Diagnostics.message msg);
+        arrow1
     | _ ->
       let loc = mk_loc expr.pexp_loc.loc_start typ.ptyp_loc.loc_end in
       let expr = Ast_helper.Exp.constraint_ ~loc expr typ in

tests/syntax_tests/data/parsing/grammar/expressions/arrow.res

@@ -103,6 +103,8 @@ let un = (():u)
 type d<'a,'b> = ('a,'b)
 let c = (): d<'a,'b> => (1,2)
 
+let arr = (): array<nullable<int>> => []
+
 let fn = f => f;
 type f = int => unit;
 let a = fn(_ => (): f);

tests/syntax_tests/data/parsing/grammar/expressions/expected/arrow.res.txt

@@ -74,6 +74,7 @@ type nonrec u = unit
 let un = (() : u)
 type nonrec ('a, 'b) d = ('a * 'b)
 let c [arity:1]() = ((1, 2) : ('a, 'b) d)
+let arr () = ([||] : int nullable array)
 let fn [arity:1]f = f
 type nonrec f = int -> unit (a:1)
 let a = fn (fun [arity:1]_ -> () : f)

tests/syntax_tests/data/printer/expr/expected/fun.res.txt

@@ -356,3 +356,5 @@ let query = (~url, ()): (unit => unit => unit => unit) => {
 let f = a => b => a + b
 let f = (a, b) => (b, c) => a + b + c + d
 let f = (a, b) => (b, c) => (e, f, g) => a + b + c + d + e + f + g
+
+let unitConstraint = (): array<nullable<int>> => []

tests/syntax_tests/data/printer/expr/fun.res

@@ -297,3 +297,5 @@ let query = (~url, ()): (unit => unit => unit => unit) => {
 let f = (. a) => (. b) => a + b
 let f = (. a, b) => (. b, c) => a + b + c + d
 let f = (. a, b) => (. b , c) => (. e , f, g) => a + b + c + d  + e + f + g
+
+let unitConstraint = (): array<nullable<int>> => []