Fix arity error message with optional args. (rescript-lang#7284)

* Fix arity error message with optional args. Fixes rescript-lang#6637 * singular on 1 args * Update CHANGELOG.md

Author: cristianoc

CHANGELOG.md

@@ -22,6 +22,7 @@
 - Fix `%external` extension. https://github.com/rescript-lang/rescript/pull/7272
 - Fix issue with type environment for unified ops. https://github.com/rescript-lang/rescript/pull/7277
 - Fix completion for application with tagged template. https://github.com/rescript-lang/rescript/pull/7278
+- Fix error message for arity in the presence of optional arguments. https://github.com/rescript-lang/rescript/pull/7284
 
 # 12.0.0-alpha.8
 

compiler/ml/typecore.ml

@@ -3514,13 +3514,14 @@ and type_application ?type_clash_context total_app env funct (sargs : sargs) :
  | Some arity ->
  let newarity = arity - nargs in
  let fully_applied = newarity <= 0 in
- if total_app && not fully_applied then
- raise
- (Error
- ( funct.exp_loc,
- env,
- Uncurried_arity_mismatch
- (funct.exp_type, arity, List.length sargs) ));
+ (if total_app && not fully_applied then
+ let required_args = List.length sargs in
+ raise
+ (Error
+ ( funct.exp_loc,
+ env,
+ Uncurried_arity_mismatch
+ (funct.exp_type, required_args + newarity, required_args) )));
  let new_t =
  if fully_applied then new_t
  else
@@ -4463,7 +4464,7 @@ let report_error env ppf error =
  "@ @[It is applied with @{<error>%d@} argument%s but it requires \
  @{<info>%d@}.@]@]"
  args
- (if args = 0 then "" else "s")
+ (if args = 1 then "" else "s")
  arity
  | Field_not_optional (name, typ) ->
  fprintf ppf "Field @{<info>%s@} is not optional in type %a. Use without ?"

tests/build_tests/super_errors/expected/arity_mismatch.res.expected

@@ -7,4 +7,4 @@
  3 │ 
 
  This function has type (~f: 'a => 'a, unit) => int
- It is applied with 1 arguments but it requires 2.
+ It is applied with 1 argument but it requires 2.

tests/build_tests/super_errors/expected/method_arity_mismatch.res.expected

@@ -9,4 +9,4 @@
  5 │ 
 
  This function has type (int, int) => unit
- It is applied with 1 arguments but it requires 2.
+ It is applied with 1 argument but it requires 2.

tests/build_tests/super_errors/expected/moreArguments1.res.expected

@@ -7,4 +7,4 @@
  3 │ 
 
  This function has type (~a: int, ~b: int) => int
- It is applied with 1 arguments but it requires 2.
+ It is applied with 1 argument but it requires 2.

tests/build_tests/super_errors/expected/moreArguments2.res.expected

@@ -7,4 +7,4 @@
  3 │ 
 
  This function has type (int, int) => int
- It is applied with 1 arguments but it requires 2.
+ It is applied with 1 argument but it requires 2.

tests/build_tests/super_errors/expected/moreArguments3.res.expected

@@ -7,4 +7,4 @@
  3 │ 
 
  This function has type (int, int, 'a, 'b) => int
- It is applied with 1 arguments but it requires 4.
+ It is applied with 1 argument but it requires 4.

tests/build_tests/super_errors/expected/moreArguments4.res.expected

@@ -7,4 +7,4 @@
  3 │ 
 
  This function has type (int, ~b: int, ~c: 'a, ~d: 'b) => int
- It is applied with 1 arguments but it requires 4.
+ It is applied with 1 argument but it requires 4.

tests/build_tests/super_errors/expected/moreArguments5.res.expected

@@ -8,4 +8,4 @@
  6 │ 
 
  This function has type (int, 'a, 'b, 'c) => Sub.a
- It is applied with 1 arguments but it requires 4.
+ It is applied with 1 argument but it requires 4.

tests/build_tests/super_errors/expected/opt_args_arity.res.expected

@@ -0,0 +1,10 @@
+
+ We've found a bug for you!
+ /.../fixtures/opt_args_arity.res:2:9
+
+ 1 │ let f = (~a=0, b, c) => a + b + c
+ 2 │ let x = f(42)
+ 3 │ 
+
+ This function has type (~a: int=?, int, int) => int
+ It is applied with 1 argument but it requires 2.