Change Hash#compact to keep default values and compare_by_identity flag

The documentation states it returns a copy of self with nil value
entries removed.  However, the previous behavior was creating a
plain new hash with non-nil values copied into it.  This change
aligns the behavior with the documentation.

Fixes [Bug #19113]

hash.c

@@ -4307,15 +4307,6 @@ delete_if_nil(VALUE key, VALUE value, VALUE hash)
     return ST_CONTINUE;
 }
 
-static int
-set_if_not_nil(VALUE key, VALUE value, VALUE hash)
-{
-    if (!NIL_P(value)) {
-        rb_hash_aset(hash, key, value);
-    }
-    return ST_CONTINUE;
-}
-
 /*
  *  call-seq:
  *    hash.compact -> new_hash
@@ -4329,9 +4320,12 @@ set_if_not_nil(VALUE key, VALUE value, VALUE hash)
 static VALUE
 rb_hash_compact(VALUE hash)
 {
-    VALUE result = rb_hash_new();
+    VALUE result = rb_hash_dup(hash);
     if (!RHASH_EMPTY_P(hash)) {
-        rb_hash_foreach(hash, set_if_not_nil, result);
+        rb_hash_foreach(result, delete_if_nil, result);
+    }
+    else if (rb_hash_compare_by_id_p(hash)) {
+        result = rb_hash_compare_by_id(result);
     }
     return result;
 }

spec/ruby/core/hash/compact_spec.rb

@@ -18,6 +18,30 @@ describe "Hash#compact" do
     @hash.compact
     @hash.should == @initial_pairs
   end
+
+  ruby_version_is '3.3' do
+    it "retains the default value" do
+      hash = Hash.new(1)
+      hash.compact.default.should == 1
+      hash[:a] = 1
+      hash.compact.default.should == 1
+    end
+
+    it "retains the default_proc" do
+      pr = proc { |h, k| h[k] = [] }
+      hash = Hash.new(&pr)
+      hash.compact.default_proc.should == pr
+      hash[:a] = 1
+      hash.compact.default_proc.should == pr
+    end
+
+    it "retains compare_by_identity_flag" do
+      hash = {}.compare_by_identity
+      hash.compact.compare_by_identity?.should == true
+      hash[:a] = 1
+      hash.compact.compare_by_identity?.should == true
+    end
+  end
 end
 
 describe "Hash#compact!" do