diff --git a/complex.c b/complex.c
index ec60c00ed6..144a59d533 100644
--- a/complex.c
+++ b/complex.c
@@ -563,6 +563,9 @@ nucomp_f_complex(int argc, VALUE *argv, VALUE klass)
     if (!NIL_P(opts)) {
         raise = rb_opts_exception_p(opts, raise);
     }
+    if (argc > 0 && CLASS_OF(a1) == rb_cComplex && a2 == Qundef) {
+        return a1;
+    }
     return nucomp_convert(rb_cComplex, a1, a2, raise);
 }
 
diff --git a/ext/bigdecimal/bigdecimal.c b/ext/bigdecimal/bigdecimal.c
index 652f341fb9..6d5047553b 100644
--- a/ext/bigdecimal/bigdecimal.c
+++ b/ext/bigdecimal/bigdecimal.c
@@ -2722,6 +2722,9 @@ f_BigDecimal(int argc, VALUE *argv, VALUE self)
     Real *pv;
     VALUE obj;
 
+    if (argc > 0 && CLASS_OF(argv[0]) == rb_cBigDecimal) {
+        if (argc == 1 || (argc == 2 && RB_TYPE_P(argv[1], T_HASH))) return argv[0];
+    }
     obj = TypedData_Wrap_Struct(rb_cBigDecimal, &BigDecimal_data_type, 0);
     pv = BigDecimal_new(argc, argv);
     if (pv == NULL) return Qnil;
diff --git a/ext/pathname/pathname.c b/ext/pathname/pathname.c
index 70f82583a1..1cf0f90b9f 100644
--- a/ext/pathname/pathname.c
+++ b/ext/pathname/pathname.c
@@ -1323,6 +1323,8 @@ path_unlink(VALUE self)
 static VALUE
 path_f_pathname(VALUE self, VALUE str)
 {
+    if (CLASS_OF(str) == rb_cPathname)
+        return str;
     return rb_class_new_instance(1, &str, rb_cPathname);
 }
 
diff --git a/test/bigdecimal/test_bigdecimal.rb b/test/bigdecimal/test_bigdecimal.rb
index e4f14449a1..b3368b4f4b 100644
--- a/test/bigdecimal/test_bigdecimal.rb
+++ b/test/bigdecimal/test_bigdecimal.rb
@@ -54,6 +54,11 @@ class TestBigDecimal < Test::Unit::TestCase
     assert_equal(111, BigDecimal("1_1_1_"))
     assert_equal(10**(-1), BigDecimal("1E-1"), '#4825')
     assert_equal(1234, BigDecimal(" \t\n\r \r1234 \t\n\r \r"))
+    bd = BigDecimal.new("1.12", 1)
+    assert_same(bd, BigDecimal(bd))
+    assert_same(bd, BigDecimal(bd, exception: false))
+    assert_not_same(bd, BigDecimal(bd, 1))
+    assert_not_same(bd, BigDecimal(bd, 1, exception: false))
 
     assert_raise(ArgumentError) { BigDecimal("1", -1) }
     assert_raise_with_message(ArgumentError, /"1__1_1"/) { BigDecimal("1__1_1") }
diff --git a/test/pathname/test_pathname.rb b/test/pathname/test_pathname.rb
index eaecc52475..af523f9f98 100644
--- a/test/pathname/test_pathname.rb
+++ b/test/pathname/test_pathname.rb
@@ -471,6 +471,12 @@ class TestPathname < Test::Unit::TestCase
     assert_raise(ArgumentError) { Pathname.new("a\0") }
   end
 
+  def test_global_constructor
+    p = Pathname.new('a')
+    assert_equal(p, Pathname('a'))
+    assert_same(p, Pathname(p))
+  end
+
   class AnotherStringLike # :nodoc:
     def initialize(s) @s = s end
     def to_str() @s end
diff --git a/test/ruby/test_complex.rb b/test/ruby/test_complex.rb
index 2a72f3bcb9..a4fe9d4232 100644
--- a/test/ruby/test_complex.rb
+++ b/test/ruby/test_complex.rb
@@ -123,6 +123,10 @@ class Complex_Test < Test::Unit::TestCase
     assert_raise(TypeError){Complex(Object.new)}
     assert_raise(ArgumentError){Complex()}
     assert_raise(ArgumentError){Complex(1,2,3)}
+    c = Complex(1,0)
+    assert_same(c, Complex(c))
+    assert_same(c, Complex(c, exception: false))
+    assert_raise(ArgumentError){Complex(c, bad_keyword: true)}
 
     if (0.0/0).nan?
       assert_nothing_raised{Complex(0.0/0)}