reflect behavior varies depending on the presence of fmt.Println!

[mark-summerfield]

The following program should output:

-2
-2

But actually outputs

5
-2

But if you uncomment the commented fmt.Println statement it does produce the right
answers (-2, -2)!

So I guess there's a weird reflect bug.

: hg identify
d6903b7fbff4+ weekly/weekly.2011-04-13

I'm on debian testing 64-bit.

package main

import (
    "fmt"
    "reflect"
    )

func main() {
    i := M(5, 1, 2, 3, -2, 4).Int()
    fmt.Println(i)
    f := M(5.0, 1.0, 2.0, 3.0, -2.0, 4.0).Float()
    fmt.Println(f)
}

func M(x interface{}, ys ...interface{}) reflect.Value {
    m := reflect.NewValue(x)
    k := m.Kind()
    for _, y := range ys {
        //fmt.Println(m) // Uncomment this and it gives the right answer!
        v := reflect.NewValue(y)
        switch k {
        case reflect.Int: if v.Int() < m.Int() { m.Set(v) }
        case reflect.Float64: if v.Float() < m.Float() { m.Set(v) }
        }
    }
    return m
}

[rsc]

Comment 1:

Owner changed to @rsc.

Status changed to Accepted.

[gopherbot]

Comment 2 by qyzhai:

reflect: more efficient; cannot Set result of NewValue anymore
http://code.google.com/p/go/source/detail?r=47bd15b1505695064d61a4d9fa6533065f5d5e74

[rsc]

Comment 3:

This doesn't have to do with reflect, although the new
reflect changes make the program not run anymore.
I checked out the version that Mark is running on Linux
and can reproduce the problem.  It is a compiler optimization bug.
I have a workaround but I want to spend a bit more time looking
for the root cause to make sure I fix the real problem.

Status changed to Started.

[rsc]

Comment 5:

This issue was closed by revision bac8f18.

Status changed to Fixed.