cmd/compile: seemingly identical anonymous types lead to confusing error messages

[zzz125]

What version of Go are you using (go version)?

1.8rc3 (broken, also in rc1)
1.7.5 (works)

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/k/dev/go"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build538144559=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"

What did you do?

https://play.golang.org/p/rQzEbb-u6Y

also, this is my test code with modules (i cant simulate it in playground):
test.zip

to run it, set GOPATH and run it with:
$ go run src/a.go

the output for go1.7 is:
-> main.X main.X
Hello, playground {2 3}

for go1.8:
panic: interface conversion: interface {} is struct { x int; y int }, not struct { x int; y int }

it does't work in my another project either now, after switching to 1.8rc

What did you expect to see?

old behaviour as in go 1.7 where it works

What did you see instead?

panic: interface conversion: interface {} is struct { x int; y int }, not struct { x int; y int }

[randall77]

Thanks for the issue. This looks like two anonymous types (declared in two separate packages, but identical) are being given different type pointers.

Here's a cleaner repro:

a.go:

package main

import "b"

func main() {
	_ = b.GetY().(struct{ x, y int })

b/b.go:

package b

func GetY() interface{} {
	return struct{ x, y int }{2, 3}
}

GetY gets inlined, so the type test boils down to (in main.main):

 104c52f:       48 8d 05 8a e5 00 00    lea    0xe58a(%rip),%rax        # 105aac0 <runtime.rodata+0xe500>
 104c536:       48 8d 0d 03 e6 00 00    lea    0xe603(%rip),%rcx        # 105ab40 <runtime.rodata+0xe580>
 104c53d:       48 39 c8                cmp    %rcx,%rax
 104c540:       75 0a                   jne    104c54c <main.main+0x4c>

Which seems wrong. Both of those LEAs should be of the same address.
I thought we had fixed all the non-unique type problems. Guess not.

I'll look more in the morning.

@crawshaw @rsc

[crawshaw]

These are different types.

The fields x and y are unexported, and so cannot be accessed outside of package b. If you export them, X and Y, this code works as you would expect.

[zzz125]

@crawshaw

even if so, i think the error should be more clear, because the folowing is unreadable:

panic: interface conversion: interface {} is struct { x int; y int }, not struct { x int; y int }
                                                 ^------------- same thing ---------^

probably it should be
panic: interface conversion: interface {} is b. struct { x int; y int }, not main. struct { x int; y int }

[crawshaw]

A better error would be better.

[randall77]

Hm, so is the bug in 1.7 then? It ran the repro successfully.

[crawshaw]

I would say so, yes. Probably something I introduced when working on binary size reductions for 1.7. I elided some unnecessary pkgpath values, and seem to remember at least once getting overenthusiastic.

[randall77]

Repro runs successfully for all of 1.2 through 1.7...