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, 7 insertions, 0 deletions

diff --git a/NEWS.md b/NEWS.md
index d760de7c11..3fabf7b331 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -121,6 +121,13 @@ Excluding feature bug fixes.
       your plan to https://github.com/ruby/xmlrpc
       or https://github.com/ruby/net-telnet.
 
+* EXPERIMENTAL: Hash#each consistently yields a 2-element array [[Bug #12706]]
+
+    * Now `{ a: 1 }.each(&->(k, v) { })` raises an ArgumentError
+      due to lambda's arity check.
+    * This is experimental; if it brings a big incompatibility issue,
+      it may be reverted until 2.8/3.0 release.
+
 ## Stdlib compatibility issues
 
 Excluding feature bug fixes.