From: "jeremyevans0 (Jeremy Evans)" <noreply@...>
Date: 2022-03-10T17:29:52+00:00
Subject: [ruby-core:107836] [Ruby master Bug#18621] Fiber.yield loses the fact it was kwargs from Fiber#resume

Issue #18621 has been updated by jeremyevans0 (Jeremy Evans).


I don't think this is a bug, I think this should be expected behavior.  Absent use of ruby2_keywords, in no other place in Ruby 3 should you expect the following code to work, no matter than value of `args`:

```ruby
def foo(a: ) = a
foo(*args)
```


However, assuming this is considered a bug, the natural way to fix it would be to flag the hash as a ruby2_keywords hash.  That's not difficult to do, but it doesn't fix the example, because ruby2_keywords processing only occurs if splatting an array, it doesn't occur if splatting a hash.  So in order for it to work, the final line would need to be: `foo(*[args])`, or you need to change it so that multiple arguments are used:

```ruby
def foo(b, a: 1) = a

f = Fiber.new do
  args = Fiber.yield
  args
end
f.resume
args = f.resume(1, a: 1)
foo(*args)
```

The reason for this issue is that Fiber#resume does its own argument handling (`make_passing_arg` in `cont.c`), returning the passed argument instead of an array of passed arguments if passed a single argument.

Here's a diff if we still want to make this change:

```diff
diff --git a/cont.c b/cont.c
index f9ebb4483e..4d9e2e94d0 100644
--- a/cont.c
+++ b/cont.c
@@ -29,6 +29,7 @@ extern int madvise(caddr_t, size_t, int);
 #include "gc.h"
 #include "internal.h"
 #include "internal/cont.h"
+#include "internal/hash.h"
 #include "internal/proc.h"
 #include "internal/warnings.h"
 #include "ruby/fiber/scheduler.h"
@@ -2242,6 +2243,13 @@ fiber_switch(rb_fiber_t *fiber, int argc, const VALUE *argv, int kw_splat, rb_fi
     rb_context_t *cont = &fiber->cont;
     rb_thread_t *th = GET_THREAD();

+    if (kw_splat && argc >= 0) {
+        VALUE last_arg = argv[argc-1];
+        if (RB_TYPE_P(last_arg, T_HASH)) {
+            ((struct RHash *)last_arg)->basic.flags |= RHASH_PASS_AS_KEYWORDS;
+        }
+    }
+
     /* make sure the root_fiber object is available */
     if (th->root_fiber == NULL) root_fiber_alloc(th);

```

Be aware that if you go this direction, even if we remove the `ruby2_keywords` methods, the support for automatically treating hashes as keywords can never be removed, it must remain part of Ruby forever.

----------------------------------------
Bug #18621: Fiber.yield loses the fact it was kwargs from Fiber#resume
https://bugs.ruby-lang.org/issues/18621#change-96765

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.2.0dev (2022-03-03T08:56:31Z master c1790f8c11) [x86_64-linux]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: REQUIRED, 3.1: REQUIRED
----------------------------------------
```ruby
f = Fiber.new do
  args = Fiber.yield
  args
end
f.resume
args = f.resume(a: 1)
Hash.ruby2_keywords_hash?(args) # => false, but should be true, isn't it?
```

This also means if there is `foo(*args)` later and `foo` would require kwargs it would fail:

```ruby
def foo(a: 1) = a

f = Fiber.new do
  args = Fiber.yield
  args
end
f.resume
args = f.resume(a: 1)
foo(*args)
# =>
# -:1:in `foo': wrong number of arguments (given 1, expected 0) (ArgumentError)
# 	from -:9:in `<main>'
```

cc @jeremyevans0 @mame



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