diff --git a/spec/std/http/request_spec.cr b/spec/std/http/request_spec.cr
index 5c53426a0e9a..7ae9aa34957e 100644
--- a/spec/std/http/request_spec.cr
+++ b/spec/std/http/request_spec.cr
@@ -6,8 +6,8 @@ private class EmptyIO < IO
     0
   end
 
-  def write(slice : Bytes) : UInt64
-    slice.size.to_u64
+  def write(slice : Bytes) : Int64
+    slice.size.to_i64
   end
 end
 
diff --git a/spec/std/http/server/response_spec.cr b/spec/std/http/server/response_spec.cr
index ff35b45527aa..b42e537d317e 100644
--- a/spec/std/http/server/response_spec.cr
+++ b/spec/std/http/server/response_spec.cr
@@ -12,11 +12,11 @@ private class ReverseResponseOutput < IO
   def initialize(@output : IO)
   end
 
-  def write(slice : Bytes) : UInt64
+  def write(slice : Bytes) : Int64
     slice.reverse_each do |byte|
       @output.write_byte(byte)
     end
-    slice.size.to_u64
+    slice.size.to_i64
   end
 
   def read(slice : Bytes)
diff --git a/spec/std/io/io_spec.cr b/spec/std/io/io_spec.cr
index 60413fcf995a..b09c096e83ea 100644
--- a/spec/std/io/io_spec.cr
+++ b/spec/std/io/io_spec.cr
@@ -46,7 +46,7 @@ private class SimpleIOMemory < IO
     count
   end
 
-  def write(slice : Bytes) : UInt64
+  def write(slice : Bytes) : Int64
     count = slice.size
     new_bytesize = bytesize + count
     if new_bytesize > @capacity
@@ -56,7 +56,7 @@ private class SimpleIOMemory < IO
     slice.copy_to(@buffer + @bytesize, count)
     @bytesize += count
 
-    slice.size.to_u64
+    slice.size.to_i64
   end
 
   def to_slice
@@ -99,8 +99,8 @@ private class OneByOneIO < IO
     1
   end
 
-  def write(slice : Bytes) : UInt64
-    slice.size.to_u64
+  def write(slice : Bytes) : Int64
+    slice.size.to_i64
   end
 end
 
diff --git a/spec/std/io/sized_spec.cr b/spec/std/io/sized_spec.cr
index 46cb7d625e4f..a02eafd1556f 100644
--- a/spec/std/io/sized_spec.cr
+++ b/spec/std/io/sized_spec.cr
@@ -5,8 +5,8 @@ private class NoPeekIO < IO
     0
   end
 
-  def write(bytes : Bytes) : UInt64
-    0u64
+  def write(bytes : Bytes) : Int64
+    0i64
   end
 
   def peek
diff --git a/spec/support/io.cr b/spec/support/io.cr
index 2eb785856c32..9ce2cd44c944 100644
--- a/spec/support/io.cr
+++ b/spec/support/io.cr
@@ -8,10 +8,10 @@ class RaiseIOError < IO
     raise IO::Error.new("...")
   end
 
-  def write(slice : Bytes) : UInt64
+  def write(slice : Bytes) : Int64
     @writes += 1
     raise IO::Error.new("...") if @raise_on_write
-    slice.size.to_u64
+    slice.size.to_i64
   end
 
   def flush
diff --git a/src/compress/deflate/writer.cr b/src/compress/deflate/writer.cr
index b32a3af8b300..0df595422e04 100644
--- a/src/compress/deflate/writer.cr
+++ b/src/compress/deflate/writer.cr
@@ -43,16 +43,16 @@ class Compress::Deflate::Writer < IO
   end
 
   # See `IO#write`.
-  def write(slice : Bytes) : UInt64
+  def write(slice : Bytes) : Int64
     check_open
 
-    return 0u64 if slice.empty?
+    return 0i64 if slice.empty?
 
     @stream.avail_in = slice.size
     @stream.next_in = slice
     consume_output LibZ::Flush::NO_FLUSH
 
-    slice.size.to_u64
+    slice.size.to_i64
   end
 
   # See `IO#flush`.
diff --git a/src/compress/gzip/writer.cr b/src/compress/gzip/writer.cr
index 8998ee291e81..19ba228c1fb3 100644
--- a/src/compress/gzip/writer.cr
+++ b/src/compress/gzip/writer.cr
@@ -68,10 +68,10 @@ class Compress::Gzip::Writer < IO
   end
 
   # See `IO#write`.
-  def write(slice : Bytes) : UInt64
+  def write(slice : Bytes) : Int64
     check_open
 
-    return 0u64 if slice.empty?
+    return 0i64 if slice.empty?
 
     flate_io = write_header
     flate_io.write(slice)
@@ -83,7 +83,7 @@ class Compress::Gzip::Writer < IO
     # uncompressed data size can be bigger.
     @isize &+= slice.size
 
-    slice.size.to_u64
+    slice.size.to_i64
   end
 
   # Flushes data, forcing writing the gzip header if no
diff --git a/src/compress/zip/checksum_writer.cr b/src/compress/zip/checksum_writer.cr
index 179bfabdcb19..194e96debe9e 100644
--- a/src/compress/zip/checksum_writer.cr
+++ b/src/compress/zip/checksum_writer.cr
@@ -13,8 +13,8 @@ module Compress::Zip
       raise IO::Error.new "Can't read from Zip::Writer entry"
     end
 
-    def write(slice : Bytes) : UInt64
-      return 0u64 if slice.empty?
+    def write(slice : Bytes) : Int64
+      return 0i64 if slice.empty?
 
       @count += slice.size
       @crc32 = Digest::CRC32.update(slice, @crc32) if @compute_crc32
diff --git a/src/compress/zlib/writer.cr b/src/compress/zlib/writer.cr
index b4e4479314a9..f66605d0fbb6 100644
--- a/src/compress/zlib/writer.cr
+++ b/src/compress/zlib/writer.cr
@@ -44,17 +44,17 @@ class Compress::Zlib::Writer < IO
   end
 
   # See `IO#write`.
-  def write(slice : Bytes) : UInt64
+  def write(slice : Bytes) : Int64
     check_open
 
-    return 0u64 if slice.empty?
+    return 0i64 if slice.empty?
 
     write_header unless @wrote_header
 
     @flate_io.write(slice)
     @adler32 = Digest::Adler32.update(slice, @adler32)
 
-    slice.size.to_u64
+    slice.size.to_i64
   end
 
   # Flushes data, forcing writing the zlib header if no
diff --git a/src/float.cr b/src/float.cr
index 23eda5aeb44a..1303ee44888d 100644
--- a/src/float.cr
+++ b/src/float.cr
@@ -93,7 +93,7 @@ struct Float
 
   # Writes this float to the given *io* in the given *format*.
   # See also: `IO#write_bytes`.
-  def to_io(io : IO, format : IO::ByteFormat) : UInt64
+  def to_io(io : IO, format : IO::ByteFormat) : Int64
     format.encode(self, io)
   end
 
diff --git a/src/http/content.cr b/src/http/content.cr
index 09aa6b2e0017..1719a0c4c0c0 100644
--- a/src/http/content.cr
+++ b/src/http/content.cr
@@ -41,7 +41,7 @@ module HTTP
       super
     end
 
-    def skip(bytes_count : Int) : UInt64
+    def skip(bytes_count : Int) : Int64
       ensure_send_continue
       super
     end
@@ -73,7 +73,7 @@ module HTTP
       @io.peek
     end
 
-    def skip(bytes_count : Int) : UInt64
+    def skip(bytes_count : Int) : Int64
       ensure_send_continue
       @io.skip(bytes_count)
     end
@@ -164,8 +164,8 @@ module HTTP
       peek
     end
 
-    def skip(bytes_count : Int) : UInt64
-      bytes_count = bytes_count.to_u64
+    def skip(bytes_count : Int) : Int64
+      bytes_count = bytes_count.to_i64
       ensure_send_continue
 
       if bytes_count <= @chunk_remaining
diff --git a/src/http/server/response.cr b/src/http/server/response.cr
index eab4a03a7907..78f273e16c53 100644
--- a/src/http/server/response.cr
+++ b/src/http/server/response.cr
@@ -80,8 +80,8 @@ class HTTP::Server
     end
 
     # See `IO#write(slice)`.
-    def write(slice : Bytes) : UInt64
-      return 0u64 if slice.empty?
+    def write(slice : Bytes) : Int64
+      return 0i64 if slice.empty?
 
       @output.write(slice)
     end
diff --git a/src/http/web_socket/protocol.cr b/src/http/web_socket/protocol.cr
index 077e99da545b..08e6c2736058 100644
--- a/src/http/web_socket/protocol.cr
+++ b/src/http/web_socket/protocol.cr
@@ -52,8 +52,8 @@ class HTTP::WebSocket::Protocol
       @pos = 0
     end
 
-    def write(slice : Bytes) : UInt64
-      return 0u64 if slice.empty?
+    def write(slice : Bytes) : Int64
+      return 0i64 if slice.empty?
 
       count = Math.min(@buffer.size - @pos, slice.size)
       (@buffer + @pos).copy_from(slice.to_unsafe, count)
@@ -67,7 +67,7 @@ class HTTP::WebSocket::Protocol
         write(slice + count)
       end
 
-      slice.size.to_u64
+      slice.size.to_i64
     end
 
     def read(slice : Bytes)
diff --git a/src/int.cr b/src/int.cr
index ec5efb4f78bc..455892562c2a 100644
--- a/src/int.cr
+++ b/src/int.cr
@@ -638,7 +638,7 @@ struct Int
   # Writes this integer to the given *io* in the given *format*.
   #
   # See also: `IO#write_bytes`.
-  def to_io(io : IO, format : IO::ByteFormat) : UInt64
+  def to_io(io : IO, format : IO::ByteFormat) : Int64
     format.encode(self, io)
   end
 
diff --git a/src/io.cr b/src/io.cr
index df29eab4e666..f7b5d687bb7a 100644
--- a/src/io.cr
+++ b/src/io.cr
@@ -29,10 +29,10 @@ require "c/errno"
 #     slice.size
 #   end
 #
-#   def write(slice : Bytes) : UInt64
+#   def write(slice : Bytes) : Int64
 #     slice.size.times { |i| @slice[i] = slice[i] }
 #     @slice += slice.size
-#     slice.size.to_u64
+#     slice.size.to_i64
 #   end
 # end
 #
@@ -100,7 +100,7 @@ abstract class IO
   # io.write(slice)
   # io.to_s # => "abcd"
   # ```
-  abstract def write(slice : Bytes) : UInt64
+  abstract def write(slice : Bytes) : Int64
 
   # Closes this `IO`.
   #
@@ -464,7 +464,7 @@ abstract class IO
   end
 
   # Writes a slice of UTF-8 encoded bytes to this `IO`, using the current encoding.
-  def write_utf8(slice : Bytes) : UInt64
+  def write_utf8(slice : Bytes) : Int64
     if encoder = encoder()
       encoder.write(self, slice)
     else
@@ -812,8 +812,8 @@ abstract class IO
   # io.gets    # => "world"
   # io.skip(1) # raises IO::EOFError
   # ```
-  def skip(bytes_count : Int) : UInt64
-    bytes_count = bytes_count.to_u64
+  def skip(bytes_count : Int) : Int64
+    bytes_count = bytes_count.to_i64
     remaining = bytes_count
     buffer = uninitialized UInt8[4096]
     while remaining > 0
@@ -826,8 +826,8 @@ abstract class IO
 
   # Reads and discards bytes from `self` until there
   # are no more bytes.
-  def skip_to_end : UInt64
-    bytes_count = 0_u64
+  def skip_to_end : Int64
+    bytes_count = 0i64
     buffer = uninitialized UInt8[4096]
     while (len = read(buffer.to_slice)) > 0
       bytes_count &+= len
@@ -842,7 +842,7 @@ abstract class IO
   # io.write_byte 97_u8
   # io.to_s # => "a"
   # ```
-  def write_byte(byte : UInt8) : UInt64
+  def write_byte(byte : UInt8) : Int64
     x = byte
     write Slice.new(pointerof(x), 1)
   end
@@ -861,7 +861,7 @@ abstract class IO
   # io.rewind
   # io.gets(4) # => "\u{4}\u{3}\u{2}\u{1}"
   # ```
-  def write_bytes(object, format : IO::ByteFormat = IO::ByteFormat::SystemEndian) : UInt64
+  def write_bytes(object, format : IO::ByteFormat = IO::ByteFormat::SystemEndian) : Int64
     object.to_io(self, format)
   end
 
@@ -1117,12 +1117,12 @@ abstract class IO
   #
   # io2.to_s # => "hello"
   # ```
-  def self.copy(src, dst) : UInt64
+  def self.copy(src, dst) : Int64
     buffer = uninitialized UInt8[4096]
-    count = 0_u64
+    count = 0_i64
     while (len = src.read(buffer.to_slice).to_i32) > 0
       dst.write buffer.to_slice[0, len]
-      count += len
+      count &+= len
     end
     count
   end
@@ -1137,16 +1137,16 @@ abstract class IO
   #
   # io2.to_s # => "hel"
   # ```
-  def self.copy(src, dst, limit : Int) : UInt64
+  def self.copy(src, dst, limit : Int) : Int64
     raise ArgumentError.new("Negative limit") if limit < 0
 
-    limit = limit.to_u64
+    limit = limit.to_i64
 
     buffer = uninitialized UInt8[4096]
     remaining = limit
     while (len = src.read(buffer.to_slice[0, Math.min(buffer.size, Math.max(remaining, 0))])) > 0
       dst.write buffer.to_slice[0, len]
-      remaining -= len
+      remaining &-= len
     end
     limit - remaining
   end
diff --git a/src/io/buffered.cr b/src/io/buffered.cr
index f38b953f3a30..008fb466d9dc 100644
--- a/src/io/buffered.cr
+++ b/src/io/buffered.cr
@@ -110,8 +110,8 @@ module IO::Buffered
   end
 
   # :nodoc:
-  def skip(bytes_count : Int) : UInt64
-    bytes_count = bytes_count.to_u64
+  def skip(bytes_count : Int) : Int64
+    bytes_count = bytes_count.to_i64
     check_open
 
     if bytes_count <= @in_buffer_rem.size
@@ -128,18 +128,18 @@ module IO::Buffered
   end
 
   # Buffered implementation of `IO#write(slice)`.
-  def write(slice : Bytes) : UInt64
+  def write(slice : Bytes) : Int64
     # NOTE: It returns the bytes written without differencing whether
     # they are kept in the buffer or sent to the underlying IO.
     check_open
 
-    return 0u64 if slice.empty?
+    return 0i64 if slice.empty?
 
     count = slice.size
 
     if sync?
       unbuffered_write(slice)
-      return slice.size.to_u64
+      return slice.size.to_i64
     end
 
     if flush_on_newline?
@@ -156,7 +156,7 @@ module IO::Buffered
     if count >= @buffer_size
       flush
       unbuffered_write slice[0, count]
-      return slice.size.to_u64
+      return slice.size.to_i64
     end
 
     if count > @buffer_size - @out_count
@@ -166,11 +166,11 @@ module IO::Buffered
     slice.copy_to(out_buffer + @out_count, count)
     @out_count += count
 
-    slice.size.to_u64
+    slice.size.to_i64
   end
 
   # :nodoc:
-  def write_byte(byte : UInt8) : UInt64
+  def write_byte(byte : UInt8) : Int64
     check_open
 
     if sync?
@@ -187,7 +187,7 @@ module IO::Buffered
       flush
     end
 
-    1u64
+    1i64
   end
 
   # Turns on/off `IO` **write** buffering. When *sync* is set to `true`, no buffering
diff --git a/src/io/byte_format.cr b/src/io/byte_format.cr
index e6f774106bd6..a8d2d8c5a94d 100644
--- a/src/io/byte_format.cr
+++ b/src/io/byte_format.cr
@@ -33,27 +33,27 @@
 # io.to_slice # => Bytes[0x34, 0x12]
 # ```
 module IO::ByteFormat
-  abstract def encode(int : Int8, io : IO) : UInt64
-  abstract def encode(int : UInt8, io : IO) : UInt64
-  abstract def encode(int : Int16, io : IO) : UInt64
-  abstract def encode(int : UInt16, io : IO) : UInt64
-  abstract def encode(int : Int32, io : IO) : UInt64
-  abstract def encode(int : UInt32, io : IO) : UInt64
-  abstract def encode(int : Int64, io : IO) : UInt64
-  abstract def encode(int : UInt64, io : IO) : UInt64
-  abstract def encode(int : Int128, io : IO) : UInt64
-  abstract def encode(int : UInt128, io : IO) : UInt64
-
-  abstract def encode(int : Int8, bytes : Bytes) : UInt64
-  abstract def encode(int : UInt8, bytes : Bytes) : UInt64
-  abstract def encode(int : Int16, bytes : Bytes) : UInt64
-  abstract def encode(int : UInt16, bytes : Bytes) : UInt64
-  abstract def encode(int : Int32, bytes : Bytes) : UInt64
-  abstract def encode(int : UInt32, bytes : Bytes) : UInt64
-  abstract def encode(int : Int64, bytes : Bytes) : UInt64
-  abstract def encode(int : UInt64, bytes : Bytes) : UInt64
-  abstract def encode(int : Int128, bytes : Bytes) : UInt64
-  abstract def encode(int : UInt128, bytes : Bytes) : UInt64
+  abstract def encode(int : Int8, io : IO) : Int64
+  abstract def encode(int : UInt8, io : IO) : Int64
+  abstract def encode(int : Int16, io : IO) : Int64
+  abstract def encode(int : UInt16, io : IO) : Int64
+  abstract def encode(int : Int32, io : IO) : Int64
+  abstract def encode(int : UInt32, io : IO) : Int64
+  abstract def encode(int : Int64, io : IO) : Int64
+  abstract def encode(int : UInt64, io : IO) : Int64
+  abstract def encode(int : Int128, io : IO) : Int64
+  abstract def encode(int : UInt128, io : IO) : Int64
+
+  abstract def encode(int : Int8, bytes : Bytes) : Int64
+  abstract def encode(int : UInt8, bytes : Bytes) : Int64
+  abstract def encode(int : Int16, bytes : Bytes) : Int64
+  abstract def encode(int : UInt16, bytes : Bytes) : Int64
+  abstract def encode(int : Int32, bytes : Bytes) : Int64
+  abstract def encode(int : UInt32, bytes : Bytes) : Int64
+  abstract def encode(int : Int64, bytes : Bytes) : Int64
+  abstract def encode(int : UInt64, bytes : Bytes) : Int64
+  abstract def encode(int : Int128, bytes : Bytes) : Int64
+  abstract def encode(int : UInt128, bytes : Bytes) : Int64
 
   abstract def decode(int : Int8.class, io : IO)
   abstract def decode(int : UInt8.class, io : IO)
@@ -77,11 +77,11 @@ module IO::ByteFormat
   abstract def decode(int : Int128.class, bytes : Bytes)
   abstract def decode(int : UInt128.class, bytes : Bytes)
 
-  def encode(float : Float32, io : IO) : UInt64
+  def encode(float : Float32, io : IO) : Int64
     encode(float.unsafe_as(Int32), io)
   end
 
-  def encode(float : Float32, bytes : Bytes) : UInt64
+  def encode(float : Float32, bytes : Bytes) : Int64
     encode(float.unsafe_as(Int32), bytes)
   end
 
@@ -93,11 +93,11 @@ module IO::ByteFormat
     decode(Int32, bytes).unsafe_as(Float32)
   end
 
-  def encode(float : Float64, io : IO) : UInt64
+  def encode(float : Float64, io : IO) : Int64
     encode(float.unsafe_as(Int64), io)
   end
 
-  def encode(float : Float64, bytes : Bytes) : UInt64
+  def encode(float : Float64, bytes : Bytes) : Int64
     encode(float.unsafe_as(Int64), bytes)
   end
 
@@ -125,18 +125,18 @@ module IO::ByteFormat
       {% for type, i in %w(Int8 UInt8 Int16 UInt16 Int32 UInt32 Int64 UInt64 Int128 UInt128) %}
         {% bytesize = 2 ** (i // 2) %}
 
-        def self.encode(int : {{type.id}}, io : IO) : UInt64
+        def self.encode(int : {{type.id}}, io : IO) : Int64
           buffer = int.unsafe_as(StaticArray(UInt8, {{bytesize}}))
           buffer.reverse! unless SystemEndian == self
           io.write(buffer.to_slice)
-          UInt64.new({{bytesize}})
+          Int64.new({{bytesize}})
         end
 
-        def self.encode(int : {{type.id}}, bytes : Bytes) : UInt64
+        def self.encode(int : {{type.id}}, bytes : Bytes) : Int64
           buffer = int.unsafe_as(StaticArray(UInt8, {{bytesize}}))
           buffer.reverse! unless SystemEndian == self
           buffer.to_slice.copy_to(bytes)
-          UInt64.new({{bytesize}})
+          Int64.new({{bytesize}})
         end
 
         def self.decode(type : {{type.id}}.class, io : IO)
diff --git a/src/io/delimited.cr b/src/io/delimited.cr
index 70fc30f98a38..34d23c07066a 100644
--- a/src/io/delimited.cr
+++ b/src/io/delimited.cr
@@ -107,7 +107,7 @@ class IO::Delimited < IO
     read_bytes
   end
 
-  def write(slice : Bytes) : UInt64
+  def write(slice : Bytes) : Int64
     raise IO::Error.new "Can't write to IO::Delimited"
   end
 
diff --git a/src/io/encoding.cr b/src/io/encoding.cr
index 49a5c086d67a..aab1c720a0db 100644
--- a/src/io/encoding.cr
+++ b/src/io/encoding.cr
@@ -27,8 +27,8 @@ class IO
       @closed = false
     end
 
-    def write(io, slice : Bytes) : UInt64
-      bytes_written = 0u64
+    def write(io, slice : Bytes) : Int64
+      bytes_written = 0i64
       inbuf_ptr = slice.to_unsafe
       inbytesleft = LibC::SizeT.new(slice.size)
       outbuf = uninitialized UInt8[1024]
diff --git a/src/io/hexdump.cr b/src/io/hexdump.cr
index 35afd9b7727b..cec29b9a0af0 100644
--- a/src/io/hexdump.cr
+++ b/src/io/hexdump.cr
@@ -32,8 +32,8 @@ class IO::Hexdump < IO
     end
   end
 
-  def write(buf : Bytes) : UInt64
-    return 0u64 if buf.empty?
+  def write(buf : Bytes) : Int64
+    return 0i64 if buf.empty?
 
     @io.write(buf).tap do
       @output.puts buf.hexdump if @write
diff --git a/src/io/memory.cr b/src/io/memory.cr
index a72839c2cd2e..be97cfe1c64a 100644
--- a/src/io/memory.cr
+++ b/src/io/memory.cr
@@ -82,13 +82,13 @@ class IO::Memory < IO
 
   # See `IO#write(slice)`. Raises if this `IO::Memory` is non-writeable,
   # or if it's non-resizeable and a resize is needed.
-  def write(slice : Bytes) : UInt64
+  def write(slice : Bytes) : Int64
     check_writeable
     check_open
 
     count = slice.size
 
-    return 0u64 if count == 0
+    return 0i64 if count == 0
 
     new_bytesize = @pos + count
     if new_bytesize > @capacity
@@ -105,12 +105,12 @@ class IO::Memory < IO
     @pos += count
     @bytesize = @pos if @pos > @bytesize
 
-    slice.size.to_u64
+    slice.size.to_i64
   end
 
   # See `IO#write_byte`. Raises if this `IO::Memory` is non-writeable,
   # or if it's non-resizeable and a resize is needed.
-  def write_byte(byte : UInt8)
+  def write_byte(byte : UInt8) : Int64
     check_writeable
     check_open
 
@@ -129,7 +129,7 @@ class IO::Memory < IO
     @pos += 1
     @bytesize = @pos if @pos > @bytesize
 
-    nil
+    1i64
   end
 
   # :nodoc:
@@ -194,8 +194,8 @@ class IO::Memory < IO
   end
 
   # :nodoc:
-  def skip(bytes_count : Int) : UInt64
-    bytes_count = bytes_count.to_u64
+  def skip(bytes_count : Int) : Int64
+    bytes_count = bytes_count.to_i64
     check_open
 
     available = @bytesize - @pos
@@ -208,12 +208,12 @@ class IO::Memory < IO
   end
 
   # :nodoc:
-  def skip_to_end : UInt64
+  def skip_to_end : Int64
     check_open
 
     skipped = @bytesize - @pos
     @pos = @bytesize
-    skipped.to_u64
+    skipped.to_i64
   end
 
   # :nodoc:
diff --git a/src/io/multi_writer.cr b/src/io/multi_writer.cr
index 35e7682f961c..7ddf7f42b4a4 100644
--- a/src/io/multi_writer.cr
+++ b/src/io/multi_writer.cr
@@ -29,14 +29,14 @@ class IO::MultiWriter < IO
     @writers = writers.map(&.as(IO)).to_a
   end
 
-  def write(slice : Bytes) : UInt64
+  def write(slice : Bytes) : Int64
     check_open
 
-    return 0u64 if slice.empty?
+    return 0i64 if slice.empty?
 
     @writers.each { |writer| writer.write(slice) }
 
-    slice.size.to_u64
+    slice.size.to_i64
   end
 
   def read(slice : Bytes)
diff --git a/src/io/sized.cr b/src/io/sized.cr
index ae9b88cdb9fe..d5ae6e506cf3 100644
--- a/src/io/sized.cr
+++ b/src/io/sized.cr
@@ -61,8 +61,8 @@ class IO::Sized < IO
     peek
   end
 
-  def skip(bytes_count : Int) : UInt64
-    bytes_count = bytes_count.to_u64
+  def skip(bytes_count : Int) : Int64
+    bytes_count = bytes_count.to_i64
     check_open
 
     if bytes_count <= @read_remaining
diff --git a/src/io/stapled.cr b/src/io/stapled.cr
index 52c1f7fb69f1..1d30bdc2fe98 100644
--- a/src/io/stapled.cr
+++ b/src/io/stapled.cr
@@ -51,31 +51,31 @@ class IO::Stapled < IO
   end
 
   # Skips `reader`.
-  def skip(bytes_count : Int) : UInt64
+  def skip(bytes_count : Int) : Int64
     check_open
 
     @reader.skip(bytes_count)
   end
 
   # Skips `reader`.
-  def skip_to_end : UInt64
+  def skip_to_end : Int64
     check_open
 
     @reader.skip_to_end
   end
 
   # Writes a byte to `writer`.
-  def write_byte(byte : UInt8) : Nil
+  def write_byte(byte : UInt8) : Int64
     check_open
 
     @writer.write_byte(byte)
   end
 
   # Writes a slice to `writer`.
-  def write(slice : Bytes) : UInt64
+  def write(slice : Bytes) : Int64
     check_open
 
-    return 0u64 if slice.empty?
+    return 0i64 if slice.empty?
 
     @writer.write(slice)
   end
diff --git a/src/openssl/digest/digest_io.cr b/src/openssl/digest/digest_io.cr
index 2a3b002ad97a..b04942736f1f 100644
--- a/src/openssl/digest/digest_io.cr
+++ b/src/openssl/digest/digest_io.cr
@@ -42,8 +42,8 @@ module OpenSSL
       read_bytes
     end
 
-    def write(slice : Bytes) : UInt64
-      return 0u64 if slice.empty?
+    def write(slice : Bytes) : Int64
+      return 0i64 if slice.empty?
 
       if @mode.write?
         digest_algorithm.update(slice)
diff --git a/src/string/builder.cr b/src/string/builder.cr
index c117a45fec4f..7ba67dd2e029 100644
--- a/src/string/builder.cr
+++ b/src/string/builder.cr
@@ -38,8 +38,8 @@ class String::Builder < IO
     raise "Not implemented"
   end
 
-  def write(slice : Bytes) : UInt64
-    return 0u64 if slice.empty?
+  def write(slice : Bytes) : Int64
+    return 0i64 if slice.empty?
 
     count = slice.size
     new_bytesize = real_bytesize + count
@@ -50,10 +50,10 @@ class String::Builder < IO
     slice.copy_to(@buffer + real_bytesize, count)
     @bytesize += count
 
-    slice.size.to_u64
+    slice.size.to_i64
   end
 
-  def write_byte(byte : UInt8)
+  def write_byte(byte : UInt8) : Int64
     new_bytesize = real_bytesize + 1
     if new_bytesize > @capacity
       resize_to_capacity(Math.pw2ceil(new_bytesize))
@@ -63,7 +63,7 @@ class String::Builder < IO
 
     @bytesize += 1
 
-    nil
+    1i64
   end
 
   def buffer