OpenStruct: improve error message when passing wrong number of arguments.

Patch by Lisa Ugray (issue #15515)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@67556 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/ostruct.rb

@@ -213,7 +213,7 @@ class OpenStruct
     len = args.length
     if mname = mid[/.*(?==\z)/m]
       if len != 1
-        raise ArgumentError, "wrong number of arguments (#{len} for 1)", caller(1)
+        raise ArgumentError, "wrong number of arguments (given #{len}, expected 1)", caller(1)
       end
       modifiable?[new_ostruct_member!(mname)] = args[0]
     elsif len == 0 # and /\A[a-z_]\w*\z/ =~ mid #
@@ -221,6 +221,8 @@ class OpenStruct
         new_ostruct_member!(mid) unless frozen?
         @table[mid]
       end
+    elsif @table.key?(mid)
+      raise ArgumentError, "wrong number of arguments (given #{len}, expected 0)"
     else
       begin
         super

spec/ruby/library/openstruct/method_missing_spec.rb

@@ -32,10 +32,17 @@ describe "OpenStruct#method_missing when called with a method name ending in '='
 end
 
 describe "OpenStruct#method_missing when passed additional arguments" do
-  it "raises a NoMethodError" do
+  it "raises a NoMethodError when the key does not exist" do
     os = OpenStruct.new
     lambda { os.method_missing(:test, 1, 2, 3) }.should raise_error(NoMethodError)
   end
+
+  ruby_version_is "2.7" do
+    it "raises an ArgumentError when the key exists" do
+      os = OpenStruct.new(test: 20)
+      lambda { os.method_missing(:test, 1, 2, 3) }.should raise_error(ArgumentError)
+    end
+  end
 end
 
 describe "OpenStruct#method_missing when not passed any additional arguments" do