Dedicated error message for ternaries (#7804)

* dedicated error message for ternaries

* changelog

Author: zth

CHANGELOG.md

@@ -29,6 +29,7 @@
 #### :nail_care: Polish
 
 - Make parser less strict around leading attributes. https://github.com/rescript-lang/rescript/pull/7787
+- Dedicated error message for ternary type mismatch. https://github.com/rescript-lang/rescript/pull/7804
 
 #### :house: Internal
 

compiler/ml/error_message_utils.ml

@@ -93,6 +93,7 @@ type type_clash_context =
   | IfCondition
   | AssertCondition
   | IfReturn
+  | TernaryReturn
   | SwitchReturn
   | TryReturn
   | StringConcat
@@ -125,6 +126,7 @@ let context_to_string = function
   | Some (FunctionArgument _) -> "FunctionArgument"
   | Some ComparisonOperator -> "ComparisonOperator"
   | Some IfReturn -> "IfReturn"
+  | Some TernaryReturn -> "TernaryReturn"
   | Some Await -> "Await"
   | None -> "None"
 
@@ -168,6 +170,7 @@ let error_expected_type_text ppf type_clash_context =
   | Some AssertCondition -> fprintf ppf "But assertions must always be of type:"
   | Some IfReturn ->
     fprintf ppf "But this @{<info>if@} statement is expected to return:"
+  | Some TernaryReturn -> fprintf ppf "But this ternary is expected to return:"
   | Some ArrayValue ->
     fprintf ppf "But this array is expected to have items of type:"
   | Some (SetRecordField _) -> fprintf ppf "But the record field is of type:"
@@ -332,6 +335,11 @@ let print_extra_type_clash_help ~extract_concrete_typedecl ~env loc ppf
       "\n\n\
       \  @{<info>if@} expressions must return the same type in all branches \
        (@{<info>if@}, @{<info>else if@}, @{<info>else@})."
+  | Some TernaryReturn, _ ->
+    fprintf ppf
+      "\n\n\
+      \  Ternaries (@{<info>?@} and @{<info>:@}) must return the same type in \
+       both branches."
   | Some MaybeUnwrapOption, _ ->
     fprintf ppf
       "\n\n\

compiler/ml/typecore.ml

@@ -2826,13 +2826,25 @@ and type_expect_ ~context ?in_function ?(recarg = Rejected) env sexp ty_expected
         exp_env = env;
       }
   | Pexp_ifthenelse (scond, sifso, sifnot) -> (
+    (* TODO(attributes) Unify the attribute handling in the parser and rest of the compiler. *)
+    let is_ternary =
+      let rec has_ternary = function
+        | [] -> false
+        | ({Location.txt = "res.ternary"}, _) :: _ -> true
+        | _ :: rest -> has_ternary rest
+      in
+      has_ternary sexp.pexp_attributes
+    in
+    let return_context =
+      if is_ternary then Some TernaryReturn else Some IfReturn
+    in
     let cond =
       type_expect ~context:(Some IfCondition) env scond Predef.type_bool
     in
     match sifnot with
     | None ->
       let ifso =
-        type_expect ~context:(Some IfReturn) env sifso Predef.type_unit
+        type_expect ~context:return_context env sifso Predef.type_unit
       in
       rue
         {
@@ -2844,10 +2856,10 @@ and type_expect_ ~context ?in_function ?(recarg = Rejected) env sexp ty_expected
           exp_env = env;
         }
     | Some sifnot ->
-      let ifso = type_expect ~context:(Some IfReturn) env sifso ty_expected in
-      let ifnot = type_expect ~context:(Some IfReturn) env sifnot ty_expected in
+      let ifso = type_expect ~context:return_context env sifso ty_expected in
+      let ifnot = type_expect ~context:return_context env sifnot ty_expected in
       (* Keep sharing *)
-      unify_exp ~context:(Some IfReturn) env ifnot ifso.exp_type;
+      unify_exp ~context:return_context env ifnot ifso.exp_type;
       re
         {
           exp_desc = Texp_ifthenelse (cond, ifso, Some ifnot);

tests/build_tests/super_errors/expected/ternary_branch_mismatch.res.expected

@@ -0,0 +1,13 @@
+
+  We've found a bug for you!
+  /.../fixtures/ternary_branch_mismatch.res:1:24-26
+
+  1 │ let x = true ? "123" : 123
+  2 │ 
+
+  This has type: int
+  But this ternary is expected to return: string
+
+  Ternaries (? and :) must return the same type in both branches.
+  
+  You can convert int to string with Int.toString.

tests/build_tests/super_errors/fixtures/ternary_branch_mismatch.res

@@ -0,0 +1 @@
+let x = true ? "123" : 123