go.tools/ssa: Select's with > 1 case, but no default, generate unreachable block with an invalid Return

[axw]

What steps will reproduce the problem?
If possible, include a link to a program on play.golang.org.
1. Save http://play.golang.org/p/Q7tUGUqD9k as select.go
2. ssadump -build=F select.go

What is the expected output?

That the SSA form for "func f() int" should not contain the final block; it is
not reachable, and it has an invalid Return statement (a bare Return, whereas the
function has >0 results).

What do you see instead?

func f() int:
.0.entry:                                                               P:0 S:2
    t0 = select blocking [<-nil:chan int, <-nil:chan int] (index int, ok bool, int, int)
    t1 = extract t0 #0                                                  int
    t2 = t1 == 0:int                                                   bool
    if t2 goto 1.select.body else 2.select.next
.1.select.body:                                                         P:1 S:0
    return 123:int
.2.select.next:                                                         P:1 S:2
    t3 = t1 == 1:int                                                   bool
    if t3 goto 3.select.body else 4.select.next
.3.select.body:                                                         P:1 S:0
    return 456:int
.4.select.next:                                                         P:1 S:0
    return

Which compiler are you using (5g, 6g, 8g, gccgo)?

N/A

Which operating system are you using?

N/A

Which version are you using?  (run 'go version')

N/A

Please provide any additional information below.

I don't care too much about the unreachable block, but the invalid Return makes my
client code uglier than it ought to be. For now I'll treat bare Returns in functions
with >0 results as being unreachable.

[ianlancetaylor]

Comment 1:

Labels changed: added repo-tools.

Owner changed to @adonovan.

[adonovan]

Comment 2:

Was fixed with this CL:
https://golang.org/cl/46520043
https://code.google.com/p/go/source/detail?r=1d658e08a7da7005b59c01029519a0c935144043&repo=tools

Status changed to Fixed.