generics: Generic function not accepting nil as an argument

[alwindoss]

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

$ go version
go version go1.18 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN="/opt/gohome/bin"
GOCACHE="/home/alwin/.cache/go-build"
GOENV="/home/alwin/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/opt/gohome/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/opt/gohome"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.18"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/alwin/Sandbox/dev/tap/go.mod"
GOWORK=""
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-build174870830=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I ran this program. Code in Play

What did you expect to see?

For the code to run without errors

What did you see instead?

./main.go:10:20: cannot infer T (/home/alwin/Sandbox/dev/tap/main.go:14:13)

[alwindoss]

Problem is the line that is passing nil as a parameter to the generic function pass

[seankhliao]

working as intended, nil has no type.

[alwindoss]

But if there is a function with the argument of type interface{} i can always pass a nil.
any in this case is interface{} so why can't i pass nil?

[alwindoss]

I think this issue should not be closed without a discussion. The point i have raised about the ability to pass nil to an argument of type interface{} should be considered.
By not doing it perhaps we are introducing ambiguity in the way interface{} type works before generics vs after generics

[randall77]

You can always provide the type parameter explicitly, e.g. pass[*int](nil). The problem is if you just write pass(nil), what should T be? *int? []byte? It is underspecified.
If you want to pass a nil as an interface, use pass[any](nil).

[ianlancetaylor]

@alwindoss There is a difference in using a variable of interface type and using a constraint of interface type. I wouldn't call it an ambiguity. A variable of interface type can be set to nil, meaning the zero value of the interface type. However, a constraint of interface type does not accept a type argument of nil. That is because a type argument must be a type, and nil is not a type, and nil does not have a type.

[alwindoss]

Thanks @randall77 for showing how to make it work and @ianlancetaylor for explaining the difference.

I modified the program and ran and it passes. It passes because now it has a type that satisfies the constraint

Modified Code

[alwindoss]

Just a thought @ianlancetaylor
This is something very subtle. Would it make sense to have an example in this tutorial https://go.dev/doc/tutorial/generics

[ianlancetaylor]

I thought about it but I think it would be too complicated to put into the tutorial. I don't think it's something that people new to Go are likely to run into.