go/types, types2: gopls crash on method with type parameters

[KarlShing]

gopls version: v0.7.4
gopls flags:
update flags: proxy
extension version: 0.30.0
go version: devel go1.18-95b240b2cd
environment: Visual Studio Code darwin
initialization error: undefined
issue timestamp: Tue, 04 Jan 2022 06:54:46 GMT
restart history:
Tue, 04 Jan 2022 06:54:38 GMT: activation (enabled: true)

ATTENTION: PLEASE PROVIDE THE DETAILS REQUESTED BELOW.

Describe what you observed.

panic: method with type parameters

goroutine 178 [running]:
go/types.(*Checker).handleBailout(0xc0004921c0, 0xc000845688)
	  check.go:279  0x8b
panic({0x1852500, 0x1bca4b0})
	  panic.go:838  0x207
go/types.(*Checker).missingMethod(0xc0004921c0, {0x1bcfba0%3F, 0xc000522980%3F}, 0xc00014dbc0, 0x1)
	  lookup.go:376  0x48e
go/types.(*operand).assignableTo(0xc000b18980, 0xc0004921c0, {0x1bcfba0%3F, 0xc000522a80%3F}, 0xc000843a90)
	  operand.go:283  0x469
go/types.(*Checker).assignment(0xc0004921c0, 0xc000b18980, {0x1bcfba0, 0xc000522a80}, {0xc0008c40f0, 0x12})
	  assignments.go:87  0x80a
go/types.(*Checker).arguments(0xc0004921c0, 0xc000c07080, 0xc0009005c0, {0xc000726b20, 0x1, 0x1}, {0xc000726b30%3F, 0x2, 0x2}, {0xc000726b10, ...})
	  call.go:410  0xbf4
go/types.(*Checker).callExpr(0xc0004921c0, 0xc000b188c0, 0xc000c07080)
	  call.go:206  0x669
go/types.(*Checker).exprInternal(0xc0004921c0, 0xc000b188c0, {0x1bd1c10%3F, 0xc000c07080}, {0x0%3F, 0x0%3F})
	  expr.go:1470  0xa12
go/types.(*Checker).rawExpr(0xc000600000%3F, 0xc000b188c0%3F, {0x1bd1c10%3F, 0xc000c07080%3F}, {0x0%3F, 0x0%3F}, 0x0)
	  expr.go:1094  0x45
go/types.(*Checker).multiExpr(0xc000844840%3F, 0x128adb1%3F, {0x1bd1c10%3F, 0xc000c07080%3F})
	  expr.go:1600  0x35
go/types.(*Checker).exprList(0xc000844960%3F, {0xc0005ac370%3F, 0xc0004921c0%3F, 0x1bcfba0%3F}, 0x0)
	  call.go:249  0x97
go/types.(*Checker).initVars(0x10043c9%3F, {0xc0003ec1f0, 0x1, 0xc000c06c00%3F}, {0xc0005ac370, 0x1, 0x0%3F}, {0x1bd2150, 0xc000c011c0})
	  assignments.go:318  0x89
go/types.(*Checker).stmt(0xc0004921c0, 0x0, {0x1bd2150%3F, 0xc000c011c0%3F})
	  stmt.go:522  0x19ce
go/types.(*Checker).stmtList(0x100e8c5%3F, 0x0, {0xc0005ac380%3F, 0xc000901300%3F, 0x50%3F})
	  stmt.go:125  0xc9
go/types.(*Checker).funcBody(0xc0004921c0, 0xc00014d320, {0xc000b22780%3F, 0x1%3F}, 0xc000901000, 0xc00058ec30, {0x0, 0x0})
	  stmt.go:46  0x285
go/types.(*Checker).funcDecl.func1()
	  decl.go:876  0x45
go/types.(*Checker).processDelayed(0xc0004921c0, 0x0)
	  check.go:348  0x39
go/types.(*Checker).checkFiles(0xc0004921c0, {0xc0003ec018%3F, 0xc000b98000%3F, 0x2%3F})
	  check.go:301  0xcb
go/types.(*Checker).Files(...)
	  check.go:284
golang.org/x/tools/internal/lsp/cache.doTypeCheck({0x1bd32d8, 0xc0006a0000}, 0xc0002aa200, 0xc00094a1e0, 0x2, 0xc0002c11d0, 0x0%3F)
	  check.go:536  0x8b3
golang.org/x/tools/internal/lsp/cache.typeCheck({0x1bd32d8, 0xc0006a0000}, 0xc0002aa200, 0xc00094a1e0, 0x2, 0xc0002c11d0%3F)
	  check.go:313  0xe5
golang.org/x/tools/internal/lsp/cache.(*snapshot).buildPackageHandle.func1({0x1bd32d8%3F, 0xc0006a0000}, {0x1bccbe0%3F, 0xc0002aa200})
	  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
[Error - 2:54:39 PM] 

OPTIONAL: If you would like to share more information, you can attach your complete gopls logs.

NOTE: THESE MAY CONTAIN SENSITIVE INFORMATION ABOUT YOUR CODEBASE.
DO NOT SHARE LOGS IF YOU ARE WORKING IN A PRIVATE REPOSITORY.

<OPTIONAL: ATTACH LOGS HERE>

[findleyr]

This is really an issue with go/types, so I've remilestoned and labeled accordingly.

CC @griesemer

[griesemer]

@findleyr This panic can be only be reached if a method has type parameters. The go/parser does accept type parameters on methods, and it appears that go/types doesn't complain in the resolver (types2 complains in the resolver). go/types then complains in Checker.funcType but seems to collect them anyway. So it seems plausible that they make it all the way into missingMethod.

[findleyr]

Simple reproducer:

type T interface{ m[P any]() }

func _(t T) {
	var _ interface{ m[P any](); n() } = t
}

Interestingly this does not repro in types2, though there may be other codepaths that panic.

[gopherbot]

Change https://golang.org/cl/382538 mentions this issue: go/parser, go/types: don't parse type parameters on methods