go1.18-dev: panic: <nil> not an Int calling unsafe.Sizeof on a generic T

[kylelemons]

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

$ go version
go version devel go1.18-86ee892 Sun Aug 22 23:49:55 2021 +0000 windows/amd64

Does this issue reproduce with the latest release?

No -- this is a panic only in gotip related to type parameters and unsafe (yes, yes, I know)

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

go env Output

$ go env
set GO111MODULE=on
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\kyle\AppData\Local\go-build
set GOENV=C:\Users\kyle\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\kyle\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\kyle\go
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\Users\kyle\sdk\gotip
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\Users\kyle\sdk\gotip\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=devel go1.18-86ee892 Sun Aug 22 23:49:55 2021 +0000
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=0
set GOMOD=C:\Users\kyle\dev\generics_exp\go.mod
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=C:\Users\kyle\AppData\Local\Temp\go-build4275696402=/tmp/go-build -gno-record-gcc-switches
GOROOT/bin/go version: go version devel go1.18-86ee892 Sun Aug 22 23:49:55 2021 +0000 windows/amd64
GOROOT/bin/go tool compile -V: compile version devel go1.18-86ee892 Sun Aug 22 23:49:55 2021 +0000

What did you do?

go.mod:

module repro

go 1.18

repro.go:

//go:build go1.18

package repro

import (
	"unsafe"
)

func Breaks[T any](t T){
	_ = unsafe.Sizeof(t)
}

What did you expect to see?

I shouldn't be able to make the compiler panic

This seems like it might be a known issue or a pending TODO (apologies if so), but since I don't usually expect the compiler to be able to panic, I figured I'd file it just in case.

What did you see instead?

$ gotip build ./repro
# generics_exp/repro
panic: <nil> not an Int

goroutine 1 [running]:
go/constant.Int64Val({0x0, 0x0})
        C:/Users/kyle/sdk/gotip/src/go/constant/value.go:504 +0x119
cmd/compile/internal/noder.(*irgen).validateBuiltin(0xc00016a300, {0xc00012b18a, 0x6}, 0xc000159740)
        C:/Users/kyle/sdk/gotip/src/cmd/compile/internal/noder/validate.go:84 +0xd7
cmd/compile/internal/noder.(*irgen).validate(0xc00016a300, {0xe249c0, 0xc000159740})
        C:/Users/kyle/sdk/gotip/src/cmd/compile/internal/noder/validate.go:70 +0x157
cmd/compile/internal/noder.(*irgen).generate.func1({0xe249c0, 0xc000159740})
        C:/Users/kyle/sdk/gotip/src/cmd/compile/internal/noder/irgen.go:259 +0x30
cmd/compile/internal/syntax.Crawl.func1({0xe249c0, 0xc000159740})
        C:/Users/kyle/sdk/gotip/src/cmd/compile/internal/syntax/walk.go:41 +0x30
cmd/compile/internal/syntax.inspector.Visit(0xc000108c80, {0xe249c0, 0xc000159740})
        C:/Users/kyle/sdk/gotip/src/cmd/compile/internal/syntax/walk.go:24 +0x31
cmd/compile/internal/syntax.walker.node({{0xe21d60, 0xc000108c80}}, {0xe249c0, 0xc000159740})
        C:/Users/kyle/sdk/gotip/src/cmd/compile/internal/syntax/walk.go:76 +0x6b
cmd/compile/internal/syntax.walker.node({{0xe21d60, 0xc000108c80}}, {0xe248f8, 0xc000159780})
        C:/Users/kyle/sdk/gotip/src/cmd/compile/internal/syntax/walk.go:253 +0x2cd
cmd/compile/internal/syntax.walker.stmtList({{0xe21d60, 0xc000108c80}}, {0xc000108b70, 0x1, 0xc00041c0c0})
        C:/Users/kyle/sdk/gotip/src/cmd/compile/internal/syntax/walk.go:348 +0x91
cmd/compile/internal/syntax.walker.node({{0xe21d60, 0xc000108c80}}, {0xe24970, 0xc000159700})
        C:/Users/kyle/sdk/gotip/src/cmd/compile/internal/syntax/walk.go:238 +0x13f2
cmd/compile/internal/syntax.walker.node({{0xe21d60, 0xc000108c80}}, {0xe24bf0, 0xc0001678c0})
        C:/Users/kyle/sdk/gotip/src/cmd/compile/internal/syntax/walk.go:125 +0x949
cmd/compile/internal/syntax.walker.declList({{0xe21d60, 0xc000108c80}}, {0xc00041c1c0, 0x2, 0xc00014d6d8})
        C:/Users/kyle/sdk/gotip/src/cmd/compile/internal/syntax/walk.go:336 +0x91
cmd/compile/internal/syntax.walker.node({{0xe21d60, 0xc000108c80}}, {0xe24ba0, 0xc000433a40})
        C:/Users/kyle/sdk/gotip/src/cmd/compile/internal/syntax/walk.go:85 +0xfee
cmd/compile/internal/syntax.Walk(...)
        C:/Users/kyle/sdk/gotip/src/cmd/compile/internal/syntax/walk.go:57
cmd/compile/internal/syntax.Inspect(...)
        C:/Users/kyle/sdk/gotip/src/cmd/compile/internal/syntax/walk.go:18
cmd/compile/internal/syntax.Crawl({0xe24ba0, 0xc000433a40}, 0xc000108c70)
        C:/Users/kyle/sdk/gotip/src/cmd/compile/internal/syntax/walk.go:40 +0x75
cmd/compile/internal/noder.(*irgen).generate(0xc00016a300, {0xc00014e450, 0x1, 0xc0003966c0})
        C:/Users/kyle/sdk/gotip/src/cmd/compile/internal/noder/irgen.go:258 +0x1dd
cmd/compile/internal/noder.check2({0xc00014e450, 0x1, 0x1})
        C:/Users/kyle/sdk/gotip/src/cmd/compile/internal/noder/irgen.go:92 +0x16d
cmd/compile/internal/noder.LoadPackage({0xc00015c0f0, 0x1, 0x0})
        C:/Users/kyle/sdk/gotip/src/cmd/compile/internal/noder/noder.go:90 +0x365
cmd/compile/internal/gc.Main(0xcf0d38)
        C:/Users/kyle/sdk/gotip/src/cmd/compile/internal/gc/main.go:190 +0xaf3
main.main()
        C:/Users/kyle/sdk/gotip/src/cmd/compile/main.go:55 +0xdd

[tdakkota]

Seems like duplicate of #47716.

[martisch]

Closing as duplicate.