cmd/go2go: crash when using an imported constraint with an embedded constraint

[argusdusty]

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

C:\Users\argusdusty\Code\src\foo>go version
go version devel +abe4d3dce1 Tue Dec 8 17:59:47 2020 +0000 windows/amd64

Does this issue reproduce with the latest release?

Reproduces on abe4d3d

What operating system and processor architecture are you using (go env)?

go env Output

C:\Users\argusdusty\Code\src\foo>go env
set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\argusdusty\AppData\Local\go-build
set GOENV=C:\Users\argusdusty\AppData\Roaming\go\env
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\argusdusty\Code\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\argusdusty\Code
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\Gosrc
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\Gosrc\pkg\tool\windows_amd64
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=0
set GOMOD=
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -fmessage-length=0 -fdebug-prefix-map=C:\Users\ARGUSD~1\AppData\Local\Temp\go-build246512954=/tmp/go-build -gno-record-gcc-switches

What did you do?

foo/lib/lib.go2:

package lib

type Bar[T any] interface {
	comparable
	F1() T
}

func F2[T Bar[T]]() { println("Hello, World!") }

foo/foo.go2:

package main

import "foo/lib"

type Foo int

func (F Foo) F1() Foo { return F }

func main() { lib.F2[Foo]() }

C:\Users\argusdusty\Code\src\foo>go tool go2go build

What did you expect to see?

Successful compilation.

What did you see instead?

panic: runtime error: index out of range [0] with length 0

Stack Trace:

goroutine 1 [running]:
go/types.(*Checker).handleBailout(0xc00009e3c0, 0xc0000c99f8)
        C:/Gosrc/src/go/types/check.go:252 +0xa5
panic(0xcea5a0, 0xc0000a0ca0)
        C:/Gosrc/src/runtime/panic.go:969 +0x176
go/types.(*Checker).stmt.func1(0xc00009e3c0, 0xc000134050)
        C:/Gosrc/src/go/types/stmt.go:307 +0x8c
panic(0xcea5a0, 0xc0000a0ca0)
        C:/Gosrc/src/runtime/panic.go:975 +0x3e9
go/types.(*Checker).completeInterface(0xc00009e3c0, 0x0, 0xc0000e6580)
        C:/Gosrc/src/go/types/typexpr.go:917 +0x7b9
go/types.(*subster).typ(0xc0000c7618, 0xd54780, 0xc0000e6480, 0x0, 0x0)
        C:/Gosrc/src/go/types/subst.go:322 +0x19d9
go/types.(*Checker).subst(0xc00009e3c0, 0x70, 0xd54780, 0xc0000e6480, 0xc000099220, 0x1, 0xc000099220)
        C:/Gosrc/src/go/types/subst.go:240 +0x12b
go/types.(*Checker).instantiate(0xc00009e3c0, 0x69, 0xd54880, 0xc0000d2480, 0xc000088950, 0x1, 0x1, 0xc0000a2990, 0x1, 0x1, ...)
        C:/Gosrc/src/go/types/subst.go:139 +0x2aa
go/types.(*Checker).call(0xc00009e3c0, 0xc0000f0ac0, 0xc0001340f0, 0xd57100, 0xc0000b6f00, 0xa63df3)
        C:/Gosrc/src/go/types/call.go:176 +0x538
go/types.(*Checker).exprInternal(0xc00009e3c0, 0xc0000f0ac0, 0xd57100, 0xc0000b6f00, 0x0, 0x0, 0xfd44552e)
        C:/Gosrc/src/go/types/expr.go:1359 +0x2bbe
go/types.(*Checker).rawExpr(0xc00009e3c0, 0xc0000f0ac0, 0xd57100, 0xc0000b6f00, 0x0, 0x0, 0x0)
        C:/Gosrc/src/go/types/expr.go:1043 +0xc7
go/types.(*Checker).exprOrType(0xc00009e3c0, 0xc0000f0ac0, 0xd57100, 0xc0000b6f00)
        C:/Gosrc/src/go/types/expr.go:1785 +0x5c
go/types.(*Checker).call(0xc00009e3c0, 0xc0000f0ac0, 0xc0000d5450, 0xd56c00, 0xc0000d5450, 0xa7693b)
        C:/Gosrc/src/go/types/call.go:21 +0xd7
go/types.(*Checker).exprInternal(0xc00009e3c0, 0xc0000f0ac0, 0xd56c00, 0xc0000d5450, 0x0, 0x0, 0x38)
        C:/Gosrc/src/go/types/expr.go:1637 +0x1d3a
go/types.(*Checker).rawExpr(0xc00009e3c0, 0xc0000f0ac0, 0xd56c00, 0xc0000d5450, 0x0, 0x0, 0x0)
        C:/Gosrc/src/go/types/expr.go:1043 +0xc7
go/types.(*Checker).stmt(0xc00009e3c0, 0x0, 0xd56e40, 0xc0000885d0)
        C:/Gosrc/src/go/types/stmt.go:333 +0x3dbb
go/types.(*Checker).stmtList(0xc00009e3c0, 0x0, 0xc0000885e0, 0x1, 0x1)
        C:/Gosrc/src/go/types/stmt.go:125 +0xd6
go/types.(*Checker).funcBody(0xc00009e3c0, 0xc0000d2720, 0xc0000a25a0, 0x4, 0xc0000d27e0, 0xc0000b6f30, 0x0, 0x0)
        C:/Gosrc/src/go/types/stmt.go:42 +0x268
go/types.(*Checker).funcDecl.func1()
        C:/Gosrc/src/go/types/decl.go:813 +0x6e
go/types.(*Checker).processDelayed(0xc00009e3c0, 0x0)
        C:/Gosrc/src/go/types/check.go:327 +0x45
go/types.(*Checker).checkFiles(0xc00009e3c0, 0xc0000cc090, 0x1, 0x1, 0x0, 0x0)
        C:/Gosrc/src/go/types/check.go:295 +0x20d
go/types.(*Checker).Files(...)
        C:/Gosrc/src/go/types/check.go:257
go/types.(*Config).Check(0xc0000f0400, 0xc0000a2550, 0x4, 0xc0000f0240, 0xc0000cc090, 0x1, 0x1, 0xc0000d5360, 0x1, 0x1, ...)
        C:/Gosrc/src/go/types/api.go:392 +0x188
go/go2go.rewriteFilesInPath(0xc0000d2240, 0x0, 0x0, 0xd06733, 0x1, 0xc0000b6c60, 0x1, 0x3, 0x0, 0x0, ...)
        C:/Gosrc/src/go/go2go/go2go.go:94 +0x4f6
go/go2go.rewriteToPkgs(0xc0000d2240, 0x0, 0x0, 0xd06733, 0x1, 0xc0000985c0, 0xc0000b6ab0, 0xc0000b6a80, 0xc0000b6a50, 0xc0000b6a20)
        C:/Gosrc/src/go/go2go/go2go.go:46 +0x16e
go/go2go.Rewrite(...)
        C:/Gosrc/src/go/go2go/go2go.go:30
main.translate(0xc0000d2240, 0xd06733, 0x1)
        C:/Gosrc/src/cmd/go2go/translate.go:15 +0x4e
main.main()
        C:/Gosrc/src/cmd/go2go/main.go:78 +0xa0c

Based on my testing, this crash occurs when all of the following conditions are met:

1. The generic type must be from another package. The equivalent single-package code compiles/runs fine: https://go2goplay.golang.org/p/Y6GbuLmMw0-
2. The generic type must contain at least one embedded constraint. Any embedded constraint will do, even an empty one.
3. The generic type must contain at least one function that references the type parameter. Changing F1() T to F1() int and similarly adjusting Foo will run fine, but F1(T), F1([]T), F1() []T, etc. all crash.
4. Though it isn't valid code, the Foo type doesn't need to filled in, or exist at all: func main() { lib.F2[int]() } triggers the same compiler crash.

[argusdusty]

This issue no longer occurs in the Go 1.18 Beta 1 release, so closing it.