Make Kernel#{Pathname,BigDecimal,Complex} return argument if given correct type

This is how Kernel#{Array,String,Float,Integer,Hash,Rational} work.
BigDecimal and Complex instances are always frozen, so this should
not cause backwards compatibility issues for those.  Pathname
instances are not frozen, so potentially this could cause backwards
compatibility issues by not returning a new object.

Based on a patch from Joshua Ballanco, some minor changes by me.

Fixes [Bug #7522]

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);
 }
 

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;

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);
 }
 

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") }

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

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)}