io.ReaderWriter cannot be casted to io.Writer

[Te4nick]

Describe the bug

The net.TcpConn implements io.ReaderWriter, as of https://github.com/vlang/v/blob/master/vlib/io/readerwriter.v#L6 the io.ReaderWriter contain io.Writer so it inherits all methods.
(I need both io.Reader and io.Writer capabilities in full code)
If I try to pass field of type io.ReaderWriter to field of type io.Writer I get an error:

D:\Programming\V\rpcrud [main +2 ~2 -0 !]> ../v/v run .\examples\interfaces.v
examples/interfaces.v:34:7: error: cannot implement interface `io.Writer` with a different interface `io.ReaderWriter`
   32 |             writers: [
   33 |                 InterceptorWriter{ interceptors: s.interceptors },
   34 |                 s.stream
      |                   ~~~~~~
   35 |             ]
   36 |         }
D:\Programming\V\rpcrud [main +2 ~2 -0 !]> ../v/v --version
V 0.5.0 e3616f2

Reproduction Steps

v code:

module main

import io
import net

@[heap]
pub struct Server {
pub mut:
	stream       io.ReaderWriter
	interceptors []Interceptor
}

pub fn (s &Server) writer() &ResponseWriter {
	return &ResponseWriter{
		server: s
		writer: io.MultiWriter{
			writers: [
				InterceptorWriter{ interceptors: s.interceptors },
				s.stream
			]
		}
	}
}

pub struct ResponseWriter {
pub mut:
	server &Server
	writer io.Writer
}

pub interface Interceptor {
mut:
	on_encoded_response(resp []u8)
}

fn handle_conn(mut conn net.TcpConn) {
	defer { conn.close() or {} }

	mut srv := Server{
		stream: conn
		interceptors: []
	}

	srv.writer()
}

struct InterceptorWriter {
mut:
	interceptors []Interceptor
}

fn (mut wr InterceptorWriter) write(buf []u8) !int {
	for mut interceptor in wr.interceptors {
		interceptor.on_encoded_response(buf)
	}
	return buf.len
}


fn main() {
	addr := '127.0.0.1:42228'
	mut l := net.listen_tcp(.ip, addr)!
	println('TCP JSON-RPC server on ${addr} (Content-Length framing)')

	for {
		mut c := l.accept()!
		println("Accepted")
		go handle_conn(mut c)
	}
}

python3 script I use to trigger go handle_conn(mut c):

import socket


HOST = '127.0.0.1'  # The server's hostname or IP address
PORT = 42228        # The port used by the server

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((HOST, PORT))
s.sendall("test".encode("utf-8"))        
data = s.recv(409600)
print(data.decode())

Run v code, run python script (more in Current Behavior)

Expected Behavior

io.ReaderWriter as a superset of io.Writer should be casted to io.Writer as it contains all methods of io.Writer. For now I guess compiler is uncapable of that

Current Behavior

Errors either in runtime or compile time unable to use io.ReaderWriter as io.Writer
to reproduce: run v code, get an error:

examples/interfaces.v:19:7: error: cannot implement interface `io.Writer` with a different interface `io.ReaderWriter`
   17 |             writers: [
   18 |                 InterceptorWriter{ interceptors: s.interceptors },
   19 |                 s.stream
      |                   ~~~~~~
   20 |             ]
   21 |         }

If I as cast s.stream to io.Writer and run (pub fn (s &Server) writer() &ResponseWriter replacement):

pub fn (s &Server) writer() &ResponseWriter {
	return &ResponseWriter{
		server: s
		writer: io.MultiWriter{
			writers: [
				InterceptorWriter{ interceptors: s.interceptors },
				s.stream as io.Writer
			]
		}
	}
}

I get an error:

TCP JSON-RPC server on 127.0.0.1:42228 (Content-Length framing)
Accepted
V panic: `as_cast`: cannot convert net.TcpConn to io.Writer
 v hash: e3616f2
    pid: 0000000000000C14
    tid: 00000000000031A4
C:/Users/Te4nick/AppData/Local/Temp/v_0/interfaces.01KEH09VF1D4BJPKC3VE1K48MA.tmp.c:7710: at builtin___v_panic: Backtrace
C:/Users/Te4nick/AppData/Local/Temp/v_0/interfaces.01KEH09VF1D4BJPKC3VE1K48MA.tmp.c:3211: by I_io__ReaderWriter_as_I_io__Writer     
C:/Users/Te4nick/AppData/Local/Temp/v_0/interfaces.01KEH09VF1D4BJPKC3VE1K48MA.tmp.c:10939: by main__Server_writer
C:/Users/Te4nick/AppData/Local/Temp/v_0/interfaces.01KEH09VF1D4BJPKC3VE1K48MA.tmp.c:10943: by main__handle_conn
C:/Users/Te4nick/AppData/Local/Temp/v_0/interfaces.01KEH09VF1D4BJPKC3VE1K48MA.tmp.c:3356: by main__handle_conn_thread_wrapper       
00452f8e : by ???
0044f1be : by ???
00452fc8 : by ???
7ffc7f817374 : by ???

Trying to wrap it into if s.stream is io.Writer { ... } also gives no effect.

Possible Solution

Compiler supports casting superset Interfaces to subsets Interfaces normally (io.ReaderWriter as io.Writer)

Additional Information/Context

No response

V version

V 0.5.0 e6e28f1

Environment details (OS name and version, etc.)

V full version	V 0.5.0 e3616f2.e6e28f1
OS	windows, Windows 10 Pro 19045 64-bit
Processor	32 cpus, 64bit, little endian, Intel(R) Core(TM) i9-14900KF
Memory	17.03GB/31.77GB
V executable	D:\Programming\V\v\v.exe
V last modified time	2026-01-09 09:15:53
V home dir	OK, value: D:\Programming\V\v
VMODULES	OK, value: C:\Users\Te4nick.vmodules
VTMP	OK, value: C:\Users\Te4nick\AppData\Local\Temp\v_0
Current working dir	OK, value: D:\Programming\V\rpcrud
Git version	git version 2.45.1.windows.1
V git status	weekly.2026.02-24-ge6e28f16
.git/config present	true
cc version	N/A
gcc version	gcc (tdm64-1) 10.3.0
clang version	N/A
msvc version	N/A
tcc version	tcc version 0.9.27 (x86_64 Windows)
tcc git status	N/A
emcc version	N/A
glibc version	N/A

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[dy-tea]

Works on latest V (V 0.5.0 02a6293) with as io.Writer:

module main

import io
import net

@[heap]
pub struct Server {
pub mut:
	stream       io.ReaderWriter
	interceptors []Interceptor
}

pub fn (s &Server) writer() &ResponseWriter {
	return &ResponseWriter{
		server: s
		writer: io.MultiWriter{
			writers: [
				InterceptorWriter{
					interceptors: s.interceptors
				},
				s.stream as io.Writer,
			]
		}
	}
}

pub struct ResponseWriter {
pub mut:
	server &Server
	writer io.Writer
}

pub interface Interceptor {
mut:
	on_encoded_response(resp []u8)
}

fn handle_conn(mut conn net.TcpConn) {
	defer { conn.close() or {} }

	mut srv := Server{
		stream:       conn
		interceptors: []
	}

	srv.writer()
}

struct InterceptorWriter {
mut:
	interceptors []Interceptor
}

fn (mut wr InterceptorWriter) write(buf []u8) !int {
	for mut interceptor in wr.interceptors {
		interceptor.on_encoded_response(buf)
	}
	return buf.len
}

fn main() {
	addr := '127.0.0.1:42228'
	mut l := net.listen_tcp(.ip, addr)!
	println('TCP JSON-RPC server on ${addr} (Content-Length framing)')

	for {
		mut c := l.accept()!
		println('Accepted')
		go handle_conn(mut c)
	}
}