go/types, types: gopls crash in unification

[virtuald]

gopls version: v0.7.5
gopls flags:
update flags: proxy
extension version: 0.31.0
go version: devel go1.18-a5c0b19
environment: Visual Studio Code darwin
initialization error: undefined
issue timestamp: Mon, 31 Jan 2022 05:48:23 GMT
restart history:
Sun, 30 Jan 2022 07:58:26 GMT: activation (enabled: true)
Mon, 31 Jan 2022 04:48:18 GMT: manual (enabled: true)
Mon, 31 Jan 2022 04:49:33 GMT: installation (enabled: true)
Mon, 31 Jan 2022 04:49:39 GMT: manual (enabled: true)
Mon, 31 Jan 2022 05:35:21 GMT: manual (enabled: true)
Mon, 31 Jan 2022 05:36:46 GMT: manual (enabled: true)
Mon, 31 Jan 2022 05:43:01 GMT: manual (enabled: true)
Mon, 31 Jan 2022 05:43:44 GMT: manual (enabled: true)
Mon, 31 Jan 2022 05:45:49 GMT: manual (enabled: true)
Mon, 31 Jan 2022 05:46:28 GMT: manual (enabled: true)
Mon, 31 Jan 2022 05:47:49 GMT: manual (enabled: true)

ATTENTION: PLEASE PROVIDE THE DETAILS REQUESTED BELOW.

Describe what you observed.

panic: assertion failed

goroutine 4561 [running]:
go/types.(*Checker).handleBailout(0xc0021fbdc0, 0xc0066ef688)
	  check.go:304  0x8b
panic({0x1854660, 0x1bce580})
	  panic.go:838  0x207
go/types.assert(...)
	  errors.go:21
go/types.(*unifier).nify(0xc0034b8320, {0x1bd3b00%3F, 0xc000520e70%3F}, {0x1bd3b00%3F, 0xc0005210a0%3F}, 0x0)
	  unify.go:461  0x1045
go/types.(*unifier).unify(...)
	  unify.go:64
go/types.(*Checker).infer(0xc0021fbdc0, {0x1bd0280, 0xc002f43300}, {0xc004d09240, 0x3, 0x4}, {0x0%3F, 0x18877a0%3F, 0xc005b74900%3F}, 0xc006dcf668, ...)
	  infer.go:193  0x599
go/types.(*Checker).arguments(0xc0021fbdc0, 0xc002f43300, 0xc007ed8fc0, {0x0, 0x0, 0x0}, {0xc0069943f8%3F, 0x1, 0x1}, {0x0, ...})
	  call.go:390  0x945
go/types.(*Checker).callExpr(0xc0021fbdc0, 0xc002e44fc0, 0xc002f43300)
	  call.go:210  0x669
go/types.(*Checker).exprInternal(0xc0021fbdc0, 0xc002e44fc0, {0x1bd5b70%3F, 0xc002f43300}, {0x0%3F, 0x0%3F})
	  expr.go:1483  0x9f2
go/types.(*Checker).rawExpr(0x100eb05%3F, 0xc0066edad8%3F, {0x1bd5b70%3F, 0xc002f43300%3F}, {0x0%3F, 0x0%3F}, 0x0)
	  expr.go:1111  0x45
go/types.(*Checker).stmt(0xc0021fbdc0, 0x0, {0x1bd5d20%3F, 0xc0016155b0%3F})
	  stmt.go:394  0x247c
go/types.(*Checker).stmtList(0xc0021fbdc0%3F, 0x0, {0xc0016155c0%3F, 0xc0021fbdc0%3F, 0x1bd1c00%3F})
	  stmt.go:125  0xc9
go/types.(*Checker).funcBody(0xc0021fbdc0, 0xc006b890e0, {0x2%3F, 0xc006dcfad0%3F}, 0xc002e44e40, 0xc001f40a80, {0x0, 0x0})
	  stmt.go:46  0x285
go/types.(*Checker).exprInternal.func1()
	  expr.go:1210  0x45
go/types.(*Checker).processDelayed(0xc0021fbdc0, 0x29)
	  check.go:375  0x39
go/types.(*Checker).stmt(0xc0021fbdc0, 0x0, {0x1bd60b0%3F, 0xc003ece820%3F})
	  stmt.go:936  0x28e5
go/types.(*Checker).stmtList(0xc0021fbdc0%3F, 0x0, {0xc001a39400%3F, 0xc0021fbdc0%3F, 0x1bd1c00%3F})
	  stmt.go:125  0xc9
go/types.(*Checker).funcBody(0xc0021fbdc0, 0xc006b890e0, {0x1bd3ab0%3F, 0xc006dcf728%3F}, 0xc007ed9ac0, 0xc001f40ae0, {0x0, 0x0})
	  stmt.go:46  0x285
go/types.(*Checker).funcDecl.func1()
	  decl.go:790  0x45
go/types.(*Checker).processDelayed(0xc0021fbdc0, 0x0)
	  check.go:375  0x39
go/types.(*Checker).checkFiles(0xc0021fbdc0, {0xc00531ee88%3F, 0xc00324ab40%3F, 0x2%3F})
	  check.go:326  0xcb
go/types.(*Checker).Files(...)
	  check.go:309
golang.org/x/tools/internal/lsp/cache.doTypeCheck({0x1bd7238, 0xc0046eff00}, 0xc0091b6500, 0xc001603860, 0x2, 0xc001bb0c00, 0x1bd6620%3F)
	  check.go:536  0x8b3
golang.org/x/tools/internal/lsp/cache.typeCheck({0x1bd7238, 0xc0046eff00}, 0xc0091b6500, 0xc001603860, 0x2, 0xc001bb0c00%3F)
	  check.go:313  0xe5
golang.org/x/tools/internal/lsp/cache.(*snapshot).buildPackageHandle.func1({0x1bd7238%3F, 0xc0046eff00}, {0x1bd0b40%3F, 0xc0091b6500})
	  check.go:124  0x233
golang.org/x/tools/internal/memoize.(*Handle).run.func1()
	  memoize.go:327  0xa9
created by golang.org/x/tools/internal/memoize.(*Handle).run
	  memoize.go:320  0x1b8
[Info  - 12:48:04 AM] 

It crashes pretty consistently when I restart with this file loaded:

 package main

 import "fmt"
 
 type RC[G any, RG any] interface {
 	~[]RG
 }
 
 type RG[G any] struct{}
 
 type RSC[G any] []*RG[G]
 
 type M[Rc RC[G, RG], G any, RG any] struct {
 	Fn func(Rc)
 }

 type NFn[Rc RC[G, RG], G any, RG any] func(Rc)

-func NC[Rc RC[G, RG], G any, RG any](nFn NFn[Rc, RG]) {
+func NC[Rc RC[G, RG], G any, RG any](nFn NFn[Rc, G, RG]) {
 	var empty Rc
 	nFn(empty)
 }
 
 func NSG[G any](c RSC[G]) {
 	fmt.Println(c)
 }
 
 func MMD[Rc RC[RG], RG any, G any]() M[Rc, RG] {
 
 	var nFn NFn[Rc, RG]
 
 	var empty Rc
 	switch any(empty).(type) {
 	case BC:
 
 	case RSC[G]:
 		nFn = NSG[G]
 	}
 
 	return M[Rc, RG]{
 		Fn: func(rc Rc) {
 			NC(nFn)
 		},
 	}
 
 	return M[Rc, RG]{}
 }

In particular, the crash has something to do with adding that G marked in the diff. Without it, no crash. With it, crashes every time.

[findleyr]

CC @griesemer

I have not yet reproduced this, but the Go version here is recent (https://go.dev/cl/360615), so I don't think this crash is fixed.

That call stack indicates that this crash can occur during normal type checking.

[findleyr]

Confirmed that this repros in go/types without gopls. Here is a smaller repro:

package p

type RC[G any, RG any] interface {}

type RG[G any] struct{}

type NFn[Rc RC[G, RG], G any, RG any] func(Rc)

func NC[Rc RC[G, RG], G any, RG any](nFn NFn[Rc, G, RG]) {
}

func _[Rc RC[RG], RG any]() {
	var nFn NFn[Rc, RG]
	NC(nFn)
}

[findleyr]

Ok, minimal repro:

package p

type F[A, B any] int

func G[A, B any](F[A, B]) {
}

func _() {
	var x F[int]
	G(x)
}

[griesemer]

Thanks, @findleyr . I will take this on.

[virtuald]

Thanks @findleyr I had tried to make my example more minimal but the error kept going away. Glad you were able to make it smaller!

[findleyr]

@virtuald thanks very much for filing the report, and no need to make it perfectly minimal. It's really helpful to get crashers in any form.

In this case the type of x is invalid (with an incorrect number of type arguments), and it looks like the type checker asserts that such a type can never get to unification.

[gopherbot]

Change https://golang.org/cl/382076 mentions this issue: go/types, types2: fix unification code for defined types