hash.c: Do not use the fast path (rb_yield_values) for lambda blocks

As a semantics, Hash#each yields a 2-element array (pairs of keys and
values).  So, `{ a: 1 }.each(&->(k, v) { })` should raise an exception
due to lambda's arity check.
However, the optimization that avoids Array allocation by using
rb_yield_values for blocks whose arity is more than 1 (introduced at
b9d29603375d17c3d1d609d9662f50beaec61fa1 and some commits), seemed to
overlook the lambda case, and wrongly allowed the code above to work.

This change experimentally attempts to make it strict; now the code
above raises an ArgumentError.  This is an incompatible change; if the
compatibility issue is bigger than our expectation, it may be reverted
(until Ruby 3.0 release).

[Bug #12706]

1 files changed, 6 insertions, 0 deletions

diff --git a/test/ruby/test_hash.rb b/test/ruby/test_hash.rb
index 33d1383ed7..cc13e3f6a5 100644
--- a/test/ruby/test_hash.rb
+++ b/test/ruby/test_hash.rb
@@ -1841,4 +1841,10 @@ class TestHash < Test::Unit::TestCase
     h[obj2] = true
     assert_equal true, h[obj]
   end
+
+  def test_bug_12706
+    assert_raise(ArgumentError) do
+      {a: 1}.each(&->(k, v) {})
+    end
+  end
 end