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
b9d2960337 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]

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.

hash.c

@@ -3105,7 +3105,7 @@ static VALUE
 rb_hash_each_pair(VALUE hash)
 {
     RETURN_SIZED_ENUMERATOR(hash, 0, 0, hash_enum_size);
-    if (rb_block_arity() > 1)
+    if (rb_block_pair_yield_optimizable())
 	rb_hash_foreach(hash, each_pair_i_fast, 0);
     else
 	rb_hash_foreach(hash, each_pair_i, 0);
@@ -4446,7 +4446,7 @@ rb_hash_any_p(int argc, VALUE *argv, VALUE hash)
 	    /* yields pairs, never false */
 	    return Qtrue;
 	}
-	if (rb_block_arity() > 1)
+        if (rb_block_pair_yield_optimizable())
 	    rb_hash_foreach(hash, any_p_i_fast, (VALUE)args);
 	else
 	    rb_hash_foreach(hash, any_p_i, (VALUE)args);
@@ -5500,7 +5500,7 @@ env_each_pair(VALUE ehash)
     }
     FREE_ENVIRON(environ);
 
-    if (rb_block_arity() > 1) {
+    if (rb_block_pair_yield_optimizable()) {
 	for (i=0; i<RARRAY_LEN(ary); i+=2) {
 	    rb_yield_values(2, RARRAY_AREF(ary, i), RARRAY_AREF(ary, i+1));
 	}

internal/proc.h

@@ -17,6 +17,7 @@ struct rb_iseq_struct;          /* in vm_core.h */
 /* proc.c */
 VALUE rb_proc_location(VALUE self);
 st_index_t rb_hash_proc(st_index_t hash, VALUE proc);
+int rb_block_pair_yield_optimizable(void);
 int rb_block_arity(void);
 int rb_block_min_max_arity(int *max);
 VALUE rb_block_to_s(VALUE self, const struct rb_block *block, const char *additional_info);

proc.c

@@ -1144,6 +1144,41 @@ block_setup(struct rb_block *block, VALUE block_handler)
     }
 }
 
+int
+rb_block_pair_yield_optimizable(void)
+{
+    int min, max;
+    const rb_execution_context_t *ec = GET_EC();
+    rb_control_frame_t *cfp = ec->cfp;
+    VALUE block_handler = rb_vm_frame_block_handler(cfp);
+    struct rb_block block;
+
+    if (block_handler == VM_BLOCK_HANDLER_NONE) {
+	rb_raise(rb_eArgError, "no block given");
+    }
+
+    block_setup(&block, block_handler);
+    min = rb_vm_block_min_max_arity(&block, &max);
+
+    switch (vm_block_type(&block)) {
+      case block_handler_type_symbol:
+        return 0;
+
+      case block_handler_type_proc:
+	{
+	    VALUE procval = block_handler;
+	    rb_proc_t *proc;
+	    GetProcPtr(procval, proc);
+            if (proc->is_lambda) return 0;
+            if (min != max) return 0;
+            return min > 1;
+	}
+
+      default:
+        return min > 1;
+    }
+}
+
 int
 rb_block_arity(void)
 {

spec/ruby/core/hash/shared/each.rb

@@ -21,19 +21,37 @@ describe :hash_each, shared: true do
     ary.sort.should == ["a", "b", "c"]
   end
 
-  it "yields 2 values and not an Array of 2 elements when given a callable of arity 2" do
-    obj = Object.new
-    def obj.foo(key, value)
-      ScratchPad << key << value
+  ruby_version_is ""..."2.8" do
+    it "yields 2 values and not an Array of 2 elements when given a callable of arity 2" do
+      obj = Object.new
+      def obj.foo(key, value)
+        ScratchPad << key << value
+      end
+
+      ScratchPad.record([])
+      { "a" => 1 }.send(@method, &obj.method(:foo))
+      ScratchPad.recorded.should == ["a", 1]
+
+      ScratchPad.record([])
+      { "a" => 1 }.send(@method, &-> key, value { ScratchPad << key << value })
+      ScratchPad.recorded.should == ["a", 1]
     end
+  end
 
-    ScratchPad.record([])
-    { "a" => 1 }.send(@method, &obj.method(:foo))
-    ScratchPad.recorded.should == ["a", 1]
+  ruby_version_is "2.8" do
+    it "yields an Array of 2 elements when given a callable of arity 2" do
+      obj = Object.new
+      def obj.foo(key, value)
+      end
 
-    ScratchPad.record([])
-    { "a" => 1 }.send(@method, &-> key, value { ScratchPad << key << value })
-    ScratchPad.recorded.should == ["a", 1]
+      -> {
+        { "a" => 1 }.send(@method, &obj.method(:foo))
+      }.should raise_error(ArgumentError)
+
+      -> {
+        { "a" => 1 }.send(@method, &-> key, value { })
+      }.should raise_error(ArgumentError)
+    end
   end
 
   it "uses the same order as keys() and values()" do

struct.c

@@ -821,7 +821,7 @@ rb_struct_each_pair(VALUE s)
 
     RETURN_SIZED_ENUMERATOR(s, 0, 0, struct_enum_size);
     members = rb_struct_members(s);
-    if (rb_block_arity() > 1) {
+    if (rb_block_pair_yield_optimizable()) {
 	for (i=0; i<RSTRUCT_LEN(s); i++) {
 	    VALUE key = rb_ary_entry(members, i);
 	    VALUE value = RSTRUCT_GET(s, i);

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