[ruby-core:79451] [Ruby trunk Bug#13196] Improve keyword argument errors when non-keyword arguments given

Issue #13196 has been updated by Nobuyoshi Nakada.


How about this?

```
$ ./miniruby -e 'def explode(code:)end' -e 'explode(1)'
-e:1:in `explode': wrong number of arguments (given 1, expected 0 with required keyword code) (ArgumentError)
	from -e:2:in `<main>'
```

```diff
diff --git a/vm_args.c b/vm_args.c
index 6cded80924..76516f60e0 100644
--- a/vm_args.c
+++ b/vm_args.c
@@ -725,7 +725,25 @@ raise_argument_error(rb_thread_t *th, const rb_iseq_t *iseq, const VALUE exc)
 static void
 argument_arity_error(rb_thread_t *th, const rb_iseq_t *iseq, const int miss_argc, const int min_argc, const int max_argc)
 {
-    raise_argument_error(th, iseq, rb_arity_error_new(miss_argc, min_argc, max_argc));
+    VALUE exc = rb_arity_error_new(miss_argc, min_argc, max_argc);
+    if (iseq->body->param.flags.has_kw) {
+	const struct rb_iseq_param_keyword *const kw = iseq->body->param.keyword;
+	const ID *keywords = kw->table;
+	int req_key_num = kw->required_num;
+	if (req_key_num > 0) {
+	    VALUE mesg = rb_attr_get(exc, idMesg);
+	    rb_str_resize(mesg, RSTRING_LEN(mesg)-1);
+	    rb_str_cat_cstr(mesg, " with required keyword");
+	    if (req_key_num > 1) rb_str_cat_cstr(mesg, "s");
+	    do {
+		rb_str_cat_cstr(mesg, " ");
+		rb_str_append(mesg, rb_id2str(*keywords++));
+		rb_str_cat_cstr(mesg, ",");
+	    } while (--req_key_num);
+	    RSTRING_PTR(mesg)[RSTRING_LEN(mesg)-1] = ')';
+	}
+    }
+    raise_argument_error(th, iseq, exc);
 }
 
 static void
```

----------------------------------------
Bug #13196: Improve keyword argument errors when non-keyword arguments given
https://bugs.ruby-lang.org/issues/13196#change-62883

* Author: Olivier Lacan
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
----------------------------------------
Given the following method definition:


```ruby
def explode(code:)
  puts "Boom!"
end
```

If a Ruby user doesn't provide any arguments when calling the `explode` method, the following helpful feedback is given:

```ruby
explode
ArgumentError: missing keyword: code
```

But when a Ruby user mistakenly provides a regular argument, the exception message is obtuse and unhelpful: 

```ruby
explode "1234"
ArgumentError: wrong number of arguments (given 1, expected 0)
```

This does not provide information to properly recover from the error. Worse, it's incorrect. It is not true that the method expected 0 arguments. The method expected 1 keyword argument.

Instead, Ruby should respond something like: 

```ruby
explode "1234"
ArgumentError: missing keyword: code, given "1234" which is not a keyword argument.
```

One could argue that this situation would call for a different error class, perhaps a `KeywordArgumentError` that would inherit from `ArgumentError`, but that would extend the scope of this feature request a bit too far in my mind. 



-- 
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