cmd/compile: crash on interface embedding cycles if a type alias is used

[zigo101]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.10rc2 linux/amd64

What did you do?

package main

type I = interface {
	I2
}

type I2 interface {
	I
}

func main() {
}

What did you expect to see?

Compiler doesn't crash. An error message should be printed.

What did you see instead?

Compiler crashes.

# command-line-arguments
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x70 pc=0x9fec2c]

goroutine 1 [running]:
...

[mvdan]

Slightly smaller repro:

package p

type I = interface {
        I2
}

type I2 interface {
        I
}

If one removes the =, the compiler properly detects the recursive type and errors. It seems like it doesn't when a type alias is involved.

[gopherbot]

Change https://golang.org/cl/93955 mentions this issue: cmd/compile: fix crash on interface alias cycle

[mvdan]

This is turning out to be a subtle bug in the compiler, and I've spent enough hours failing to find the fix. I'll let someone else (@mdempsky?) take over.

As a piece of perhaps useful information, if you replace the interface types with structs, there is no crash. So the bug involves both aliases and interfaces - if either of them is removed, the bug disappears.

[griesemer]

Possibly related to #18640.

[gopherbot]

Change https://golang.org/cl/118078 mentions this issue: cmd/compile: correct alias cycle detection