cmd/compile: panic on invalid struct initialization

[rogpeppe]

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

go version devel +b59b42c Tue Jul 17 10:07:18 2018 +0000 linux/amd64

What did you do?

Compile this:

package main

type A struct {
	X int
}

type B struct {
	*A
}

func main() {
	_ = B{X: 0}
}

What did you expect to see?

./bug.go:12:9: unknown field 'X' in struct literal of type B

What did you see instead?

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0xb964d8]

goroutine 1 [running]:
cmd/compile/internal/gc.typecheckcomplit(0xc000360380, 0x0)
	/home/rog/go/src/cmd/compile/internal/gc/typecheck.go:3155 +0x14f8
cmd/compile/internal/gc.typecheck1(0xc000360380, 0x2, 0xc00031b0e0)
	/home/rog/go/src/cmd/compile/internal/gc/typecheck.go:859 +0x4ee1
cmd/compile/internal/gc.typecheck(0xc000360380, 0x2, 0xc00031b0e0)
	/home/rog/go/src/cmd/compile/internal/gc/typecheck.go:238 +0x6ad
cmd/compile/internal/gc.typecheckas(0xc000360300)
	/home/rog/go/src/cmd/compile/internal/gc/typecheck.go:3336 +0xa4
cmd/compile/internal/gc.typecheck1(0xc000360300, 0x1, 0xcb547b)
	/home/rog/go/src/cmd/compile/internal/gc/typecheck.go:1985 +0x3735
cmd/compile/internal/gc.typecheck(0xc000360300, 0x1, 0x3)
	/home/rog/go/src/cmd/compile/internal/gc/typecheck.go:238 +0x6ad
cmd/compile/internal/gc.typecheckslice(0xc00000c5f0, 0x1, 0x1, 0x1)
	/home/rog/go/src/cmd/compile/internal/gc/typecheck.go:68 +0x50
cmd/compile/internal/gc.Main(0xcd4b28)
	/home/rog/go/src/cmd/compile/internal/gc/main.go:518 +0x2059
main.main()
	/home/rog/go/src/cmd/compile/main.go:51 +0x96

[gopherbot]

Change https://golang.org/cl/124395 mentions this issue: cmd/compile: fix crash on invalid struct literal

[odeke-em]

This issue is actually a regression, on Go1.10 it returns the expected error as per
https://play.golang.org/p/WqHsGsJTeAs

whereas on tip it trips out as reported

[odeke-em]

Confirmed that this was introduced by CL 97076 aka commit 42ecf39#diff-7728aae3914e92bdec496b35e670668c /cc @ChrisALiles @mdempsky

@mvdan has a CL prepared, great!

[mdempsky]

@odeke-em Thanks for identifying the regression root cause.