cmd/compile: internal compiler error: unexpected Interface for <nil>

[eliasnaur]

Please answer these questions before submitting your issue. Thanks!

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

Playground

Does this issue reproduce with the latest release?

Yes

What did you do?

Ran https://play.golang.org/p/0Cq3cm9-bo

What did you expect to see?

main.go:4:13: undefined: G

What did you see instead?

tmp/sandbox815323628/main.go:4:13: undefined: G
tmp/sandbox815323628/main.go:6:5: internal compiler error: unexpected Interface for <nil>

goroutine 1 [running]:
runtime/debug.Stack(0x0, 0x0, 0x0)
	/usr/local/go/src/runtime/debug/stack.go:24 +0xa7
cmd/compile/internal/gc.Fatalf(0xb654a1, 0x1b, 0xc4202fc0e8, 0x1, 0x1)
	/usr/local/go/src/cmd/compile/internal/gc/subr.go:181 +0x230
cmd/compile/internal/gc.Val.Interface(0x0, 0x0, 0xc4202fc2e8, 0xc4202fc318)
	/usr/local/go/src/cmd/compile/internal/gc/const.go:99 +0x2df
cmd/compile/internal/gc.checkDupExprCases(0xc4202cee80, 0xc42000c548, 0x1, 0x1)
	/usr/local/go/src/cmd/compile/internal/gc/swt.go:628 +0x5ad
cmd/compile/internal/gc.typecheckswitch(0xc4202ceb80)
	/usr/local/go/src/cmd/compile/internal/gc/swt.go:210 +0x13a8
cmd/compile/internal/gc.typecheck1(0xc4202ceb80, 0x1, 0xc4202cea80)
	/usr/local/go/src/cmd/compile/internal/gc/typecheck.go:2157 +0xc871
cmd/compile/internal/gc.typecheck(0xc4202ceb80, 0x1, 0xc4202cea80)
	/usr/local/go/src/cmd/compile/internal/gc/typecheck.go:201 +0x764
cmd/compile/internal/gc.typecheckslice(0xc4202c7b40, 0x3, 0x4, 0x1)
	/usr/local/go/src/cmd/compile/internal/gc/typecheck.go:55 +0x54
cmd/compile/internal/gc.Main(0xb73f98)
	/usr/local/go/src/cmd/compile/internal/gc/main.go:480 +0x1fd0
main.main()
	/usr/local/go/src/cmd/compile/main.go:49 +0x95

[mvdan]

The good news is that this is already fixed: #21988

The bad news is that it's a regression in 1.9 not deemed important enough for a 1.9.x. And I would imagine the playground has to run stable, not tip.

[mdempsky]

Duplicate of #21988.