cmd/compile: compiler does not allow "direct binding" of function return values to copy arguments #15992
Description
go version devel +2f08888 Tue Jun 7 06:04:23 2016 +0000 linux/amd64
The spec allows "forwarding"/"direct binding" of return values to function arguments
https://golang.org/ref/spec#Calls
As a special case, if the return values of a function or method g are equal in number and individually
assignable to the parameters of another function or method f, then the call f(g(parameters_of_g)) will
invoke f after binding the return values of g to the parameters of f in order.
For the following program (https://play.golang.org/p/YEJSEfQLA8):
package main
import (
"fmt"
)
func f(a []byte) ([]byte, []byte) {
return a, []byte("abc")
}
func g(a []byte) ([]byte, string) {
return a, "abc"
}
func main() {
a := []byte{1, 2, 3}
n := copy(f(a))
fmt.Println(n, a)
b := []byte{1, 2, 3}
n = copy(f(b))
fmt.Println(n, b)
}the compiler issues messages
prog.go:17: missing arguments to copy
prog.go:21: missing arguments to copy
The same program passes when compiled via gotypes and gccgo.