Array: fix range assignment index out of bounds

spec/std/array_spec.cr

@@ -379,6 +379,30 @@ describe "Array" do
       a[nil..2] = [6, 7]
       a.should eq([6, 7, 4, 5])
     end
+
+    it "replaces entire range with a value for empty array (#8341)" do
+      a = [] of Int32
+      a[..] = 6
+      a.should eq([6])
+    end
+
+    it "pushes a new value with []=(...)" do
+      a = [1, 2, 3]
+      a[3..] = 4
+      a.should eq([1, 2, 3, 4])
+    end
+
+    it "replaces entire range with an array for empty array (#8341)" do
+      a = [] of Int32
+      a[..] = [1, 2, 3]
+      a.should eq([1, 2, 3])
+    end
+
+    it "concats a new array with []=(...)" do
+      a = [1, 2, 3]
+      a[3..] = [4, 5, 6]
+      a.should eq([1, 2, 3, 4, 5, 6])
+    end
   end
 
   describe "values_at" do

src/array.cr

@@ -400,7 +400,13 @@ class Array(T)
   def []=(index : Int, count : Int, value : T)
     raise ArgumentError.new "Negative count: #{count}" if count < 0
 
-    index = check_index_out_of_bounds index
+    index += size if index < 0
+
+    # We allow index == size because the range to replace
+    # can start at exactly the end of the array.
+    # So, we can't use check_index_out_of_bounds.
+    raise IndexError.new unless 0 <= index <= size
+
     count = index + count <= size ? count : size - index
 
     case count
@@ -456,7 +462,13 @@ class Array(T)
   def []=(index : Int, count : Int, values : Array(T))
     raise ArgumentError.new "Negative count: #{count}" if count < 0
 
-    index = check_index_out_of_bounds index
+    index += size if index < 0
+
+    # We allow index == size because the range to replace
+    # can start at exactly the end of the array.
+    # So, we can't use check_index_out_of_bounds.
+    raise IndexError.new unless 0 <= index <= size
+
     count = index + count <= size ? count : size - index
     diff = values.size - count