package main

import "os"

func main() {
    x := os.Stdout
    x.WriteString("foo\n")
}

When I compile this with SSA, I get:

    0x0013 00019 (tmp.go:3) MOVQ    os.Stdout(SB), AX

Note the line number. It's the line number of the import statement, not the line number where os.Stdout is mentioned.

This bug comes from earlier in the pipeline than the SSA backend. On input to SSA, we have:

.   AS u(2) l(6) colas(true) tc(1)
.   .   NAME-main.x u(1) a(true) g(1) l(6) x(0+0) class(PAUTO) f(1) tc(1) used(true) PTR64-*os.File
.   .   NAME-os.Stdout u(1) a(true) l(3) x(0+0) class(PEXTERN) tc(1) used(true) PTR64-*os.File

Note the l(3) on the os.Stdout reference.

I suspect the old compiler ends up using the line number of the AS, so it is ok, whereas the SSA compiler uses the line number for the NAME.

Probably a dup of #14742

See issue #15427

@davecheney