Make Array#pop and Array#shift raise when empty

bootstrap/spec/std/array_spec.cr

@@ -225,20 +225,6 @@ describe "Array" do
     end
   end
 
-  describe "shift" do
-    it "shifts when non empty" do
-      a = [1, 2, 3]
-      a.shift.should eq(1)
-      a.should eq([2, 3])
-    end
-
-    it "shifts when empty" do
-      a = [] of Int32
-      a.shift.should be_nil
-      a.should eq([] of Int32)
-    end
-  end
-
   describe "unshift" do
     assert do
       a = [2, 3]
@@ -313,4 +299,36 @@ describe "Array" do
     b = [1, 2, [3]]
     a.hash.should eq(b.hash)
   end
+
+  describe "pop" do
+    it "pops when non empty" do
+      a = [1, 2, 3]
+      a.pop.should eq(3)
+      a.should eq([1, 2])
+    end
+
+    it "raises when empty" do
+      begin
+        ([] of Int32).pop
+        fail "expected to raise Array::IndexOutOfBounds"
+      rescue Array::IndexOutOfBounds
+      end
+    end
+  end
+
+  describe "shift" do
+    it "shifts when non empty" do
+      a = [1, 2, 3]
+      a.shift.should eq(1)
+      a.should eq([2, 3])
+    end
+
+    it "raises when empty" do
+      begin
+        ([] of Int32).shift
+        fail "expected to raise Array::IndexOutOfBounds"
+      rescue Array::IndexOutOfBounds
+      end
+    end
+  end
 end

std/array.cr

@@ -89,13 +89,13 @@ class Array(T)
   end
 
   def pop
-    return nil if @length == 0
+    raise IndexOutOfBounds.new if @length == 0
     @length -= 1
     @buffer[@length]
   end
 
   def shift
-    return nil if @length == 0
+    raise IndexOutOfBounds.new if @length == 0
     value = @buffer[0]
     @length -=1
     @buffer.memmove(@buffer + 1, @length)