cmd/compile: internal compiler error: panic: nil

[jake-ciolek]

Go version

go 1.26.1

Output of go env in your module/workspace:

GOARCH=arm64

What did you do?

package main

type A *C
type C = D[*H]
type D[_ any] struct{}
type H D[C]

func main() {}

What did you see happen?

The following panic happened checking types near:
	./main.go:6:6
	./main.go:4:6
	./main.go:3:6
<unknown line number>: internal compiler error: panic: nil

Please file a bug report including a short program that triggers the error.
https://go.dev/issue/new

What did you expect to see?

No ICE.

[mwriter]

type D[_ any] struct{}
type C = D[*H]
type H D[C]
type A *C

I wonder how you find that at all :)

[temdenes]

I have investigated this ICE and found the root cause in cmd/compile/internal/types2/typestring.go. The typeWriter.typ function panics when it encounters a cycle during type instantiation if a context is present.

I have a working fix that provides a placeholder string instead of panicking, which allows the compiler to complete type checking. I am submitting a PR with the fix and a regression test in test/fixedbugs shortly.

[gopherbot]

Change https://go.dev/cl/758062 mentions this issue: fix: cmd/compile/internal/types2: avoid panic on recursive generic types

[gabyhelp]

Related Issues

• cmd/compile: internal compiler error (panic: nil) with self-referencing type alias #77837 (closed)
• cmd/compile: bad ptr to array in slice go.shape.*uint8 #78297
• cmd/compile: internal compiler error: declared pointer receiver type #78295
• cmd/compile: compiler crash with circular (but valid) generic alias #50779 (closed)
• cmd/compile: <autogenerated>:1: internal compiler error: panic: runtime error: invalid memory address or nil pointer dereference #54671 (closed)
• cmd/compile: nil pointer dereference #57715 (closed)
• cmd/compile: panic on invalid generic append with type parameter spread #78163 (closed)
• cmd/compile: panic for cyclic generic interfaces #56045
• cmd/compile: "panic: assertion failed" when compiling illegal code #47883 (closed)
• cmd/compile: internal compiler error: panic: interface conversion: ir.Node is *ir.IndexExpr, not *ir.StarExpr #77832 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[thepudds]

This is probably not too helpful, but FWIW, I ran a git bisect that seemed to point to:

d28b27c go/types, types2: use nil to represent incomplete explicit aliases

That's for the specific internal compiler error: panic: nil.

Prior to that commit, I think it fails with internal compiler error: unexpected types2.Invalid, which I think at least previously was a somewhat broad error (appearing for example in #72887 and various other issues), and you can also see unexpected types2.Invalid printed for the example in this issue on the playground if you use Go 1.25 (playground link).

CC @mrkfrmn @griesemer