Alias ENV.merge! as ENV.update

[Feature #15947]

Closes: https://github.com/ruby/ruby/pull/2246

hash.c

@@ -5788,6 +5788,8 @@ env_update_i(VALUE key, VALUE val)
  * call-seq:
  *   ENV.update(hash)                                        -> Hash
  *   ENV.update(hash) { |name, old_value, new_value| block } -> Hash
+ *   ENV.merge!(hash)                                        -> Hash
+ *   ENV.merge!(hash) { |name, old_value, new_value| block } -> Hash
  *
  * Adds the contents of +hash+ to the environment variables.  If no block is
  * specified entries with duplicate keys are overwritten, otherwise the value
@@ -6059,6 +6061,7 @@ Init_Hash(void)
     rb_define_singleton_method(envtbl, "invert", env_invert, 0);
     rb_define_singleton_method(envtbl, "replace", env_replace, 1);
     rb_define_singleton_method(envtbl, "update", env_update, 1);
+    rb_define_singleton_method(envtbl, "merge!", env_update, 1);
     rb_define_singleton_method(envtbl, "inspect", env_inspect, 0);
     rb_define_singleton_method(envtbl, "rehash", env_none, 0);
     rb_define_singleton_method(envtbl, "to_a", env_to_a, 0);

spec/ruby/core/env/merge_spec.rb

@@ -0,0 +1,8 @@
+require_relative '../../spec_helper'
+require_relative 'shared/update'
+
+ruby_version_is "2.7" do
+  describe "ENV.merge!" do
+    it_behaves_like :env_update, :merge!
+  end
+end

spec/ruby/core/env/shared/update.rb

@@ -0,0 +1,21 @@
+describe :env_update, shared: true do
+  it "adds the parameter hash to ENV" do
+    ENV["foo"].should == nil
+    ENV.send @method, "foo" => "bar"
+    ENV["foo"].should == "bar"
+    ENV.delete "foo"
+  end
+
+  it "yields key, the old value and the new value when replacing entries" do
+    ENV.send @method, "foo" => "bar"
+    ENV["foo"].should == "bar"
+    ENV.send(@method, "foo" => "boo") do |key, old, new|
+      key.should == "foo"
+      old.should == "bar"
+      new.should == "boo"
+      "rab"
+    end
+    ENV["foo"].should == "rab"
+    ENV.delete "foo"
+  end
+end

spec/ruby/core/env/update_spec.rb

@@ -1,25 +1,6 @@
 require_relative '../../spec_helper'
+require_relative 'shared/update'
 
 describe "ENV.update" do
-
-  it "adds the parameter hash to ENV" do
-    ENV["foo"].should == nil
-    ENV.update "foo" => "bar"
-    ENV["foo"].should == "bar"
-    ENV.delete "foo"
-  end
-
-  it "yields key, the old value and the new value when replacing entries" do
-    ENV.update "foo" => "bar"
-    ENV["foo"].should == "bar"
-    ENV.update("foo" => "boo") do |key, old, new|
-      key.should == "foo"
-      old.should == "bar"
-      new.should == "boo"
-      "rab"
-    end
-    ENV["foo"].should == "rab"
-    ENV.delete "foo"
-  end
-
+  it_behaves_like :env_update, :update
 end