rb_ary_sum: don't enter fast path if initial isn't a native numeric type.

[Bug #19530]

If the initial value isn't one of the special cased types, we directly
jump to the slow path.

array.c

@@ -8181,6 +8181,12 @@ rb_ary_sum(int argc, VALUE *argv, VALUE ary)
 
     n = 0;
     r = Qundef;
+
+    if (!FIXNUM_P(v) && !RB_BIGNUM_TYPE_P(v) && !RB_TYPE_P(v, T_RATIONAL)) {
+        i = 0;
+        goto init_is_a_value;
+    }
+
     for (i = 0; i < RARRAY_LEN(ary); i++) {
         e = RARRAY_AREF(ary, i);
         if (block_given)
@@ -8265,6 +8271,7 @@ rb_ary_sum(int argc, VALUE *argv, VALUE ary)
     }
 
     goto has_some_value;
+    init_is_a_value:
     for (; i < RARRAY_LEN(ary); i++) {
         e = RARRAY_AREF(ary, i);
         if (block_given)

spec/ruby/core/array/sum_spec.rb

@@ -10,6 +10,10 @@ describe "Array#sum" do
     [1, 2, 3].sum { |i| i * 10 }.should == 60
   end
 
+  it "doesn't apply the block init" do
+    [1, 2, 3].sum(1) { |i| i * 10 }.should == 61
+  end
+
   # https://bugs.ruby-lang.org/issues/12217
   # https://github.com/ruby/ruby/blob/master/doc/ChangeLog/ChangeLog-2.4.0#L6208-L6214
   it "uses Kahan's compensated summation algorithm for precise sum of float numbers" do
@@ -69,6 +73,15 @@ describe "Array#sum" do
     a.should_receive(:+).with(b).and_return(42)
     [b].sum(a).should == 42
   end
+
+  ruby_bug '#19530', ''...'3.3' do
+    it "calls + on the init value" do
+      a = mock("a")
+      b = mock("b")
+      a.should_receive(:+).with(42).and_return(b)
+      [42].sum(a).should == b
+    end
+  end
 end
 
 describe "Array#sum" do