gccgo: internal compiler error for (invalid) program using interface aliases

[griesemer]

$ gccgo --version
gccgo (google-gccgo-257955) 8.0.1 20180223 (experimental)
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ cat x.go
package main

type T = interface {
	m() interface{ T }
}

type S struct{}

func (S) m() interface{ T } {
	return nil
}

func main() {
	var s S
	var t T = s
	t.m()
}

$ gccgo x.go
gccgo: internal compiler error: Segmentation fault signal terminated program go1
Please submit a full bug report,
with preprocessed source if appropriate.
See <http://go.ext.google.com/go/> for instructions.

For reference:
cmd/compile reports:

x.go:3:6: invalid recursive type alias T
	x.go:3:6: T uses <T>
	x.go:3:10: <T> uses <T>
	x.go:4:3: <T> uses <T>
	x.go:4:6: <T> uses T

go/types reports (incorrectly - see issue #25301 ):

x.go:9:25: invalid AST: T is not a named type
x.go:4:17: invalid AST: T is not a named type
x.go:4:17: invalid AST: T is not a named type

The following code:

package main

type T = interface {
	m() interface{ T }
}

func main() {
}

is silently accepted by gccgo, but the embedded interface T cannot be "expanded out". We don't specify this in the spec (yet), but it's one of the limitations we have for type aliases, at least according to the design doc.

[ianlancetaylor]

CC @thanm

[thanm]

I'll take a look later today.

[benshi001]

Though this issue has the same root cause as #25851, different solutions are needed. We should rise an error like "recursive interface alias" for this issue, and generate right code for #25851.

[gopherbot]

Change https://golang.org/cl/163298 mentions this issue: compiler: traverse inherited interface aliases