go/types: panic "assertion failed" when autocompleting generic test code

[jayconrod]

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

$ go version
go version devel go1.18-8486ced8b0 Mon Aug 23 17:51:41 2021 +0000 linux/amd64

golang.org/x/tools/gopls v0.7.1
    golang.org/x/tools/gopls@v0.7.1 h1:Mh3Z8Xcoq3Zy7ksSlwDV/nzQSbjFf06A+L+F8YHq55U=

Does this issue reproduce with the latest release?

no, just master as of today

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/jay/.cache/go-build"
GOENV="/home/jay/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/jay/.cache/gomodcache"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/jay/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/jay/Code/goroot"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/jay/Code/goroot/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="devel go1.18-8486ced8b0 Mon Aug 23 17:51:41 2021 +0000"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/jay/Code/goroot/src/cmd/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build4033947821=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Started parameterizing cmd/go/internal/par.Cache. See CL 344513.

This happened as I was adding type arguments to the cache variable in cache_test.go. The file looked like this:

// Copyright 2021 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package par

import "testing"

func TestCache(t *testing.T) {
	var cache Cache[int, ]

	n := 1
	v := cache.Do(1, func() interface{} { n++; return n })
	if v != 2 {
		t.Fatalf("cache.Do(1) did not run f")
	}
	v = cache.Do(1, func() interface{} { n++; return n })
	if v != 2 {
		t.Fatalf("cache.Do(1) ran f again!")
	}
	v = cache.Do(2, func() interface{} { n++; return n })
	if v != 3 {
		t.Fatalf("cache.Do(2) did not run f")
	}
	v = cache.Do(1, func() interface{} { n++; return n })
	if v != 2 {
		t.Fatalf("cache.Do(1) did not returned saved value from original cache.Do(1)")
	}
}

What did you expect to see?

nothing exciting

What did you see instead?

gopls crashed with the panic below:

panic: assertion failed [recovered]
	panic: assertion failed

goroutine 12647 [running]:
go/types.(*Checker).handleBailout(0xc00040cb40, 0xc011ef7690)
	/home/jay/Code/goroot/src/go/types/check.go:244 +0x8b
panic({0xc0cca0, 0xe6da40})
	/home/jay/Code/goroot/src/runtime/panic.go:814 +0x214
go/types.assert(...)
	/home/jay/Code/goroot/src/go/types/errors.go:20
go/types.(*unifier).nify(0xc00d87f440, {0xe841d8, 0xc00b6ebd40}, {0xe841d8, 0xc00d87f3b0}, 0x0)
	/home/jay/Code/goroot/src/go/types/unify.go:437 +0xe56
go/types.(*unifier).nify(0xc00d87f440, {0xe84200, 0xc009b0e970}, {0xe84200, 0xc009b0f480}, 0x0)
	/home/jay/Code/goroot/src/go/types/unify.go:325 +0x825
go/types.(*unifier).unify(...)
	/home/jay/Code/goroot/src/go/types/unify.go:60
go/types.(*Checker).infer(0xc00040cb40, {0xe71d20, 0xc01115ef50}, {0xc009b0e960, 0x2, 0x4b018}, {0x0, 0x4b029, 0x40ec8b}, 0xc0107b6408, ...)
	/home/jay/Code/goroot/src/go/types/infer.go:156 +0x688
go/types.(*Checker).selector(0xc00040cb40, 0xc00f6c9880, 0xc00f4453f8)
	/home/jay/Code/goroot/src/go/types/call.go:547 +0x1a93
go/types.(*Checker).exprInternal(0xc00040cb40, 0xc00f6c9880, {0xe867a0, 0xc00f4453f8}, {0x0, 0x0})
	/home/jay/Code/goroot/src/go/types/expr.go:1364 +0xc32
go/types.(*Checker).rawExpr(0xc006024618, 0x0, {0xe867a0, 0xc00f4453f8}, {0x0, 0x0})
	/home/jay/Code/goroot/src/go/types/expr.go:1075 +0x3a
go/types.(*Checker).exprOrType(0x1000000000000d0, 0x7fb11240f668, {0xe867a0, 0xc00f4453f8})
	/home/jay/Code/goroot/src/go/types/expr.go:1557 +0x32
go/types.(*Checker).callExpr(0xc00040cb40, 0xc00f6c9880, 0xc00f4fc200)
	/home/jay/Code/goroot/src/go/types/call.go:88 +0x2e5
go/types.(*Checker).exprInternal(0xc00040cb40, 0xc00f6c9880, {0xe86200, 0xc00f4fc200}, {0x0, 0x0})
	/home/jay/Code/goroot/src/go/types/expr.go:1408 +0x945
go/types.(*Checker).rawExpr(0xc00f6c9880, 0x0, {0xe86200, 0xc00f4fc200}, {0x0, 0x0})
	/home/jay/Code/goroot/src/go/types/expr.go:1075 +0x3a
go/types.(*Checker).multiExpr(0xd0, 0x203003, {0xe86200, 0xc00f4fc200})
	/home/jay/Code/goroot/src/go/types/expr.go:1538 +0x32
go/types.(*Checker).exprList(0x10, {0xc009b0e240, 0xc0015ec0f8, 0xc0015ec158}, 0x0)
	/home/jay/Code/goroot/src/go/types/call.go:213 +0x97
go/types.(*Checker).initVars(0xc3c940, {0xc002193e60, 0x1, 0x0}, {0xc009b0e240, 0xc00ab3ab40, 0x415545}, 0x0)
	/home/jay/Code/goroot/src/go/types/assignments.go:235 +0x7b
go/types.(*Checker).shortVarDecl(0xc00040cb40, {0xe730a0, 0xc00ef9b158}, {0xc009b0e1e0, 0x1, 0x0}, {0xc009b0e240, 0x3c2b0, 0x1})
	/home/jay/Code/goroot/src/go/types/assignments.go:367 +0x9c5
go/types.(*Checker).stmt(0xc00040cb40, 0x0, {0xe86080, 0xc00f4fc240})
	/home/jay/Code/goroot/src/go/types/stmt.go:429 +0x16a7
go/types.(*Checker).stmtList(0xe84188, 0x0, {0xc00658cc00, 0x0, 0x1})
	/home/jay/Code/goroot/src/go/types/stmt.go:130 +0xc9
go/types.(*Checker).funcBody(0xc00040cb40, 0xc00f1b9260, {0xc00040cb40, 0x7fb1124be998}, 0xc00f4fde80, 0xc00ab3ba10, {0x0, 0x0})
	/home/jay/Code/goroot/src/go/types/stmt.go:46 +0x259
go/types.(*Checker).funcDecl.func1()
	/home/jay/Code/goroot/src/go/types/decl.go:786 +0x45
go/types.(*Checker).processDelayed(0xc00040cb40, 0x0)
	/home/jay/Code/goroot/src/go/types/check.go:298 +0x35
go/types.(*Checker).checkFiles(0xc00040cb40, {0xc00f4fc880, 0xc01b6de370, 0x2})
	/home/jay/Code/goroot/src/go/types/check.go:266 +0xcb
go/types.(*Checker).Files(...)
	/home/jay/Code/goroot/src/go/types/check.go:249
golang.org/x/tools/internal/lsp/cache.doTypeCheck({0xe890d8, 0xc00f2c7ec0}, 0xc010b6c100, 0xc0085fa870, 0x2, 0xc00aebf3b0, 0xc0000320c0)
	/home/jay/.cache/gomodcache/golang.org/x/tools@v0.1.6-0.20210802203754-9b21a8868e16/internal/lsp/cache/check.go:544 +0x893
golang.org/x/tools/internal/lsp/cache.typeCheck({0xe890d8, 0xc00f2c7ec0}, 0xc010b6c100, 0xc0085fa870, 0x2, 0xc00aebf3b0)
	/home/jay/.cache/gomodcache/golang.org/x/tools@v0.1.6-0.20210802203754-9b21a8868e16/internal/lsp/cache/check.go:321 +0xe5
golang.org/x/tools/internal/lsp/cache.(*snapshot).buildPackageHandle.func1({0xe890d8, 0xc00f2c7ec0}, {0xe72060, 0xc010b6c100})
	/home/jay/.cache/gomodcache/golang.org/x/tools@v0.1.6-0.20210802203754-9b21a8868e16/internal/lsp/cache/check.go:124 +0x23e
golang.org/x/tools/internal/memoize.(*Handle).run.func1()
	/home/jay/.cache/gomodcache/golang.org/x/tools@v0.1.6-0.20210802203754-9b21a8868e16/internal/memoize/memoize.go:322 +0xa9
created by golang.org/x/tools/internal/memoize.(*Handle).run
	/home/jay/.cache/gomodcache/golang.org/x/tools@v0.1.6-0.20210802203754-9b21a8868e16/internal/memoize/memoize.go:315 +0x1c5

Full output attached (too big for comment): types-assertion-failed.txt

Unfortunately, I couldn't reproduce this. Everything worked after reloading the editor window.

cc @findleyr

[findleyr]

Thanks for reporting.

CC @griesemer

[jayconrod]

The compiler didn't like my code either 😭 #47924

[griesemer]

Some of the new type-checker code is probably not as fault-tolerant yet as the old code.

[griesemer]

The failing assertion is this one (judging from revision and stack trace):

// TODO(gri) This is not always correct: two types may have the same names
//           in the same package if one of them is nested in a function.
//           Extremely unlikely but we need an always correct solution.
if x.obj.pkg == y.obj.pkg && x.obj.name == y.obj.name {
	assert(len(xargs) == len(yargs))
	for i, x := range xargs {
		if !u.nify(x, yargs[i], p) {
			return false
		}
	}
	return true
}

in unify.go for the *Named case. Maybe it's sufficient to be more tolerant when the lengths don't match. See also the TODO.

[griesemer]

@jayconrod Any chance you could play around some more with this and see if you can reproduce this? Without more input I fear this issue is not actionable.

[ianlancetaylor]

Moving milestone to 1.19.

[gopherbot]

Timed out in state WaitingForInfo. Closing.

(I am just a bot, though. Please speak up if this is a mistake or you have the requested information.)