cmd/compile: crash compiling code with local generic type

[akshaybharambe14]

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

$ go1.18beta1 version
go version go1.18beta1 windows/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go1.18beta1 env
set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\aksha\AppData\Local\go-build
set GOENV=C:\Users\aksha\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\aksha\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\aksha\go
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\Users\aksha\sdk\go1.18beta1
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\Users\aksha\sdk\go1.18beta1\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.18beta1
set GCCGO=gccgo
set GOAMD64=v1
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=D:\github.com\akshaybharambe14\go-generics-tests-bug\go.mod
set GOWORK=
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 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\aksha\AppData\Local\Temp\go-build162005594=/tmp/go-build -gno-record-gcc-switches

What did you do?

go test fails to run tests with an error associated with compiling tests.

Quick refrence

Module structure

- a.go
- a_test.go
- b.go
- b_test.go

Running go1.18beta1 test fails for this module.

As this involves multiple files, for simplicity I am providing a reference to a repository. Following are the quick steps to reproduce the problem.

git clone https://github.com/akshaybharambe14/go-generics-tests-bug
cd go-generics-tests-bug
go1.18beta1 test

What did you expect to see?

PASS
ok      github.com/akshaybharambe14/go-generics-tests-bug       0.197s

What did you see instead?

# github.com/akshaybharambe14/go-generics-tests-bug [github.com/akshaybharambe14/go-generics-tests-bug.test]
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x18 pc=0x9f5ae5]

goroutine 1 [running]:
cmd/compile/internal/noder.transformCompLit(0xc000068b80)
        c:/go/src/cmd/compile/internal/noder/transform.go:1019 +0x8a5
cmd/compile/internal/noder.(*irgen).compLit(0xc000142240, {0xc85150, 0xc0004e1000}, 0xc0003ebcc0)
        c:/go/src/cmd/compile/internal/noder/expr.go:372 +0x276
cmd/compile/internal/noder.(*irgen).expr0(0xc000142240, {0xc85150, 0xc0004e1000}, {0xc860a8?, 0xc0003ebcc0?})
        c:/go/src/cmd/compile/internal/noder/expr.go:107 +0x585
cmd/compile/internal/noder.(*irgen).expr(0xc000142240, {0xc860a8?, 0xc0003ebcc0?})
        c:/go/src/cmd/compile/internal/noder/expr.go:81 +0x5e8
cmd/compile/internal/noder.(*irgen).compLit(0xc000142240, {0xc851f0, 0xc000590450}, 0xc0003ebc70)
        c:/go/src/cmd/compile/internal/noder/expr.go:353 +0x57b
cmd/compile/internal/noder.(*irgen).expr0(0xc000142240, {0xc851f0, 0xc000590450}, {0xc860a8?, 0xc0003ebc70?})
        c:/go/src/cmd/compile/internal/noder/expr.go:107 +0x585
cmd/compile/internal/noder.(*irgen).expr(0xc000142240, {0xc860a8?, 0xc0003ebc70?})
        c:/go/src/cmd/compile/internal/noder/expr.go:81 +0x5e8
cmd/compile/internal/noder.(*irgen).compLit(0xc000142240, {0xc851c8, 0xc000091550}, 0xc0003ebc20)
        c:/go/src/cmd/compile/internal/noder/expr.go:360 +0x747
cmd/compile/internal/noder.(*irgen).expr0(0xc000142240, {0xc851c8, 0xc000091550}, {0xc860a8?, 0xc0003ebc20?})
        c:/go/src/cmd/compile/internal/noder/expr.go:107 +0x585
cmd/compile/internal/noder.(*irgen).expr(0xc000142240, {0xc860a8?, 0xc0003ebc20?})
        c:/go/src/cmd/compile/internal/noder/expr.go:81 +0x5e8
cmd/compile/internal/noder.(*irgen).exprs(0xc00007f360?, {0xc00007f350, 0x1, 0x200c00004cc80?})
        c:/go/src/cmd/compile/internal/noder/expr.go:329 +0x8e
cmd/compile/internal/noder.(*irgen).exprList(0x8?, {0xc860a8?, 0xc0003ebc20?})
        c:/go/src/cmd/compile/internal/noder/expr.go:312 +0x85
cmd/compile/internal/noder.(*irgen).stmt(0xc000142240, {0xc85f28?, 0xc0000579c0?})
        c:/go/src/cmd/compile/internal/noder/stmt.go:79 +0x418
cmd/compile/internal/noder.(*irgen).stmts(0xc00047b570?, {0xc000057a00?, 0x4, 0xc00007f648?})
        c:/go/src/cmd/compile/internal/noder/stmt.go:19 +0xaf
cmd/compile/internal/noder.(*irgen).funcBody(0xc000142240, 0xc00047c6e0, 0x93f320?, 0xc0000577c0, 0xc000057840)
        c:/go/src/cmd/compile/internal/noder/func.go:45 +0x25f
cmd/compile/internal/noder.(*irgen).funcDecl.func1()
        c:/go/src/cmd/compile/internal/noder/decl.go:145 +0xf5
cmd/compile/internal/noder.(*irgen).generate(0xc000142240, {0xc0003d4140, 0x4, 0xc0000a4000?})
        c:/go/src/cmd/compile/internal/noder/irgen.go:280 +0x227
cmd/compile/internal/noder.check2({0xc0003d4140, 0x4, 0x4})
        c:/go/src/cmd/compile/internal/noder/irgen.go:92 +0x16d
cmd/compile/internal/noder.LoadPackage({0xc00011e120, 0x4, 0x0?})
        c:/go/src/cmd/compile/internal/noder/noder.go:90 +0x335
cmd/compile/internal/gc.Main(0xb3f488)
        c:/go/src/cmd/compile/internal/gc/main.go:191 +0xb13
main.main()
        c:/go/src/cmd/compile/main.go:55 +0xdd
FAIL    github.com/akshaybharambe14/go-generics-tests-bug [build failed]

Some observations

• Test runs fine if executed in isolation (go1.18beta1 test find.go find_test.go).
• Somehow, package tests succeed by renaming one of the 'args' type declarations in tests.
  From -

func TestFind(t *testing.T) {
	type args[T comparable] struct {
		v T
		s []T
	}
}

To -

func TestFind(t *testing.T) {
	type delArgs[T comparable] struct {
		v T
		s []T
	}
}

[ianlancetaylor]

Pretty sure this is a dup of #47631.

[danscales]

This is a dup of #50177 (can't currently declare a local generic type in a non-generic function). The potential fix listed on that issue fixes this issue. We haven't yet decided if we are going to support local generic types in non-generic functions for Go 1.18, but this issue can be duped with that issue. We will either make sure things work, or issue a compile-time error in this case.

Issue #47631 is a slightly different case - support for local types (generic or non-generic) in generic functions. We will definitely not support that case in Go 1.18 (i.e. we will stay with printing a compiler error)

[danscales]

Just to clarify (I wanted to double-check myself), the problem is the two conflicting local definitions of args[T] in find_test.go and delete_test.go. We are mixing up the instantiations of the two different args[T] types. This is fixed by https://go-review.googlesource.com/c/go/+/372654/ , if we decide to check it in.