Comment 1:

FWIW, under 6g (I believe the original report was under 8g) the compiler is a little
more forthcoming
% 6g 1710.go 
1710.go:31: internal compiler error: agen: unknown op PLUS u(100) l(31) x(-1000000000)
tc(1) string

Comment 2:

Ah, ha
Here is the smallest possible program that will trigger the error
package main
func main() {
    str := ""
    for _, v := range make([]byte, 100) {
        str =+ string(v)
    }
    _ = str
}
% 6g 1710.go 
1710.go:6: internal compiler error: agen: unknown op PLUS u(100) l(6) x(-1000000000)
tc(1) string
however, if we gofmt that program, we get this
package main
func main() {
        str := ""
        for _, v := range make([]byte, 100) {
                str = +string(v)
        }
        _ = str
}
which reveals the that this isn't a compiler error, but a syntax checking error, as the
correct syntax for appending to strings is +=, no =+

Comment 3:

Ah, ha
Here is the smallest possible program that will trigger the error
package main
func main() {
    str := ""
    for _, v := range make([]byte, 100) {
        str =+ string(v)
    }
    _ = str
}
% 6g 1710.go 
1710.go:6: internal compiler error: agen: unknown op PLUS u(100) l(6) x(-1000000000)
tc(1) string
however, if we gofmt that program, we get this
package main
func main() {
        str := ""
        for _, v := range make([]byte, 100) {
                str = +string(v)
        }
        _ = str
}
which reveals the that this isn't a compiler error, but a syntax checking error, as the
correct syntax for appending to strings is +=, not =+.
http://golang.org/doc/go_spec.html#Arithmetic_operators

Comment 4:

You went and ruined the surprise.  :-(

Comment 5:

The fix should be easy: update the appropriate
table and add a test case.

Owner changed to @rsc.

Status changed to HelpWanted.

Comment 6:

> You went and ruined the surprise.  :-(
Aww shucks, sorry.

Comment 7:

This issue was closed by revision ec735c4.

Status changed to Fixed.