Fix nil pointer dereference when sending to binary channel type alias

[pk910]

Description

When sending a value to a channel that is a type alias defined in a binary package, yaegi panics with a nil pointer dereference.

This PR was created with AI assistance (Claude) as I hit this issue in a downstream project using yaegi.
The fix and tests have been reviewed and verified for correctness and functionality by myself.

Root Cause

In the send function (interp/run.go), the code directly accesses c0.typ.val to get the channel element type:

value1 := genDestValue(c0.typ.val, c1)

For source-defined channels (category chanT), .val contains the element type. But for binary channel type aliases (category valueT), .val is nil - the element type is only available via reflection in .rtype.

Fix

Use the existing elem() method which correctly handles both cases:

value1 := genDestValue(c0.typ.elem(), c1)

The elem() method (in type.go) already handles this properly:

func (t *itype) elem() *itype {
    if t.cat == valueT {
        return valueTOf(t.rtype.Elem())  // Binary types: create from reflect
    }
    return t.val  // Source types: use existing itype with all metadata
}

This is a one-line fix that fixes binary channel type aliases by using reflection to get the element type

Test Results

Test	Without Fix	With Fix
TestSendToSourceDefinedChannel	PASS	PASS
TestSendToSourceDefinedChannelTypeAlias	PASS	PASS
TestSendToBinaryChannelTypeAlias	FAIL	PASS

Minimal Reproduction

mypkg/mypkg.go

package mypkg

// IntChan is a channel type alias
type IntChan chan int

// NewIntChan creates a new IntChan
func NewIntChan() IntChan {
    return make(IntChan, 1)
}

symbols.go

package main

import (
    "reflect"
    "yaegi-repro/mypkg"
)

var Symbols = map[string]map[string]reflect.Value{}

func init() {
    Symbols["yaegi-repro/mypkg/mypkg"] = map[string]reflect.Value{
        "IntChan":    reflect.ValueOf((*mypkg.IntChan)(nil)),
        "NewIntChan": reflect.ValueOf(mypkg.NewIntChan),
    }
}

main.go

package main

import (
    "fmt"
    "log"
    "github.com/traefik/yaegi/interp"
)

func main() {
    i := interp.New(interp.Options{})
    if err := i.Use(Symbols); err != nil {
        log.Fatal(err)
    }

    code := `
package main

import "yaegi-repro/mypkg"

func main() {
    ch := mypkg.NewIntChan()
    ch <- 42  // This line panics without the fix
    val := <-ch
    println("Received:", val)
}
`
    _, err := i.Eval(code)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println("Success!")
}

Results

Without fix:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x128 pc=0x...]

goroutine 1 [running]:
github.com/traefik/yaegi/interp.(*itype).refType(0x0?, ...)
    .../interp/type.go:2065 +0x7a
github.com/traefik/yaegi/interp.(*itype).TypeOf(...)
    .../interp/type.go:2223
github.com/traefik/yaegi/interp.genDestValue(0x0, ...)
    .../interp/value.go:157 +0x25
github.com/traefik/yaegi/interp.send(...)
    .../interp/run.go:3766 +0x...

With fix:

Received: 42
Success!

[CLAassistant]

All committers have signed the CLA.

[mvertes]

The fix is correct. The test case needs to be adjusted to avoid confusion with standard library test package.

[pk910]

Heya @mvertes
Thanks for your review :)
I've updated the test accordingly.

[mvertes]

Looks good to me

[traefiker]

🚫 the mergeable state is "blocked"