cmd/go2go: decide what unsafe.Sizeof/Alignof mean when applied to values of type parameter type

[AndrewWPhillips]

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

$ go version
go version devel +893c5ec17b Thu Jul 16 21:30:46 2020 +0000 windows/amd64

Does this issue reproduce with the latest release?

N/A

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

go env Output

$ go env
set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\Andre\AppData\Local\go-build
set GOENV=C:\Users\Andre\AppData\Roaming\go\env
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\Andre\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\Andre\go
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\Users\Andre\goroot
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\Users\Andre\goroot\pkg\tool\windows_amd64
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=
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\Andrew\AppData\Local\Temp\go-build443956446=/tmp/go-build -gno-record-g
cc-switches

What did you do?

package main

import "unsafe"

type HasZero interface {
	type int, int8 // etc
}

func f(type T HasZero)() {
	const sz = unsafe.Sizeof(T(0))
}

$ go tool go2go build

What did you expect to see?

build succeeds (or perhaps build error message)

What did you see instead?

panic: Sizeof unimplemented for type sum [recovered]
panic: Sizeof unimplemented for type sum

goroutine 1 [running]:
go/types.(*Checker).handleBailout(0xc00009e3c0, 0xc0000cb9f8)
/path/goroot/src/go/types/check.go:252 +0xa5
panic(0xe21f40, 0xeb74e0)
/path/goroot/src/runtime/panic.go:969 +0x176
go/types.(*StdSizes).Sizeof(0xc0000a2280, 0xec3700, 0xc0000f0600, 0xc0000f06c0)
/path/goroot/src/go/types/sizes.go:152 +0x218
go/types.(*Config).sizeof(0xc0000f0500, 0xec3700, 0xc0000f0600, 0x0)
/path/goroot/src/go/types/sizes.go:258 +0xa2
go/types.(*Checker).builtin(0xc00009e3c0, 0xc0000f0680, 0xc0000f0440, 0x11, 0xc000020800)
/path/goroot/src/go/types/builtins.go:642 +0x3997
go/types.(*Checker).call(0xc00009e3c0, 0xc0000f0680, 0xc0000f0440, 0x30)
/path/goroot/src/go/types/call.go:63 +0xdb0
go/types.(*Checker).exprInternal(0xc00009e3c0, 0xc0000f0680, 0xebf680, 0xc0000f0440, 0x0, 0x0, 0x2b)
/path/goroot/src/go/types/expr.go:1603 +0x1df0
go/types.(*Checker).rawExpr(0xc00009e3c0, 0xc0000f0680, 0xebf680, 0xc0000f0440, 0x0, 0x0, 0x0)
/path/goroot/src/go/types/expr.go:1033 +0xc7
go/types.(*Checker).expr(0xc00009e3c0, 0xc0000f0680, 0xebf680, 0xc0000f0440)
/path/goroot/src/go/types/expr.go:1726 +0x5c
go/types.(*Checker).constDecl(0xc00009e3c0, 0xc0000d2480, 0x0, 0x0, 0xebf680, 0xc0000f0440)
/path/goroot/src/go/types/decl.go:419 +0x192
go/types.(*Checker).declStmt(0xc00009e3c0, 0xebfa00, 0xc0000f0480)
/path/goroot/src/go/types/decl.go:832 +0xe5
go/types.(*Checker).stmt(0xc00009e3c0, 0x0, 0xebf7c0, 0xc0000885d0)
/path/goroot/src/go/types/stmt.go:319 +0x3886
go/types.(*Checker).stmtList(0xc00009e3c0, 0x0, 0xc0000885e0, 0x1, 0x1)
/path/goroot/src/go/types/stmt.go:125 +0xd6
go/types.(*Checker).funcBody(0xc00009e3c0, 0xc0000d23c0, 0xfefe10, 0x1, 0xc0000d2420, 0xc0000b8e10, 0x0, 0x0)
/path/goroot/src/go/types/stmt.go:42 +0x268
go/types.(*Checker).funcDecl.func1()
/path/goroot/src/go/types/decl.go:792 +0x6e
go/types.(*Checker).processDelayed(0xc00009e3c0, 0x0)
/path/goroot/src/go/types/check.go:327 +0x45
go/types.(*Checker).checkFiles(0xc00009e3c0, 0xc0000cc0a0, 0x1, 0x1, 0x0, 0x0)
/path/goroot/src/go/types/check.go:295 +0x20d
go/types.(*Checker).Files(...)
/path/goroot/src/go/types/check.go:257
go/types.(*Config).Check(0xc0000f0500, 0xc0000a2528, 0x4, 0xc0000f0240, 0xc0000cc0a0, 0x1, 0x1, 0xc0000d5310, 0x1, 0x1, ...)
/path/goroot/src/go/types/api.go:387 +0x188
go/go2go.rewriteFilesInPath(0xc0000d2240, 0x0, 0x0, 0xe6fac3, 0x1, 0xc0000b8c60, 0x1, 0x3, 0x0, 0x0, ...)
/path/goroot/src/go/go2go/go2go.go:93 +0x4f0
go/go2go.rewriteToPkgs(0xc0000d2240, 0x0, 0x0, 0xe6fac3, 0x1, 0xc0000985c0, 0xc0000b8ab0, 0xc0000b8a80, 0xc0000b8a50, 0xc0000b8a20)
/path/goroot/src/go/go2go/go2go.go:46 +0x16e
go/go2go.Rewrite(...)
/path/goroot/src/go/go2go/go2go.go:30
main.translate(0xc0000d2240, 0xe6fac3, 0x1)
/path/goroot/src/cmd/go2go/translate.go:15 +0x4e
main.main()
/path/goroot/src/cmd/go2go/main.go:78 +0xa0c

[AndrewWPhillips]

BTW I think that having unsafe.Sizeof() work on parameter types is important. It worked (see code below) when I tried go2go a couple of months ago - using "contracts" for constraints.

$ go version
go version devel +af2b592260 Wed Apr 22 14:12:34 2020 -0700 Linux/amd64

contract (
	Element(T) {
		T int8, int16, int32, int64, int, uint8, uint16, uint32, uint64, uint //, uintptr
	}
)

func minInt(type T Element)() T {
	zero := T(0)
	if zero - 1 > 0 {
		return zero
	}
	// Handle signed types - may need fix if new types added to Go (like int128)
	switch unsafe.Sizeof(T(0)) {
	case 1:
		v := math.MinInt8
		return T(v)
	case 2:
		v := math.MinInt16
		return T(v)
	case 4:
		v := math.MinInt32
		return T(v)
	case 8:
		v := math.MinInt64
		return T(v)
	}
	panic("unknown type")
}

[ianlancetaylor]

CC @griesemer

Looks like a bug in the type checker. Perhaps unsafe.Sizeof just hasn't been implemented for type lists yet.

[griesemer]

@AndrewWPhillips It didn't "work" before - it just may not have crashed.

The question is: What does that unsafe.Sizeof(x) where x is of a type parameter type even mean? unsafe.Sizeof is a compile time constant, it must be determined when the function that contains it is compiled. I can see how we could make this work when the associated constraint contains a type list, but if there's more than one type in the type list (or they have different sizes) it's going to be very hard to maintain that "compile-time" constant aspect. We don't have any facility for something like this in the compiler at the moment, and it's not clear (to me) that we need it.

[griesemer]

I will "fix this" for now by reporting an error. We can revisit if we have a better idea of what the right approach is.

[gopherbot]

Change https://golang.org/cl/244622 mentions this issue: [dev.go2go] go/types: don't crash with unsafe.Alignof/Sizeof on a type parameter value

[griesemer]

@ianlancetaylor pointed out that one could define unsafe.Sizeof such that it would not return a compile-time constant if the argument is of type parameter type. That would certainly be trivial from a type-checker's point of view but would break an assumption we currently have for unsafe.Sizeof (but not permitting it would also break an assumption, so perhaps that's the way to go after all).

[griesemer]

Reopened and retitled for decision making.