cmd/compile: cannot slice a variable of type T constrained by [5]byte | [10]byte

[578559967]

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

$ go version
go version go1.20.3 darwin/arm64

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="arm64"
GOBIN=""
GOCACHE="/Users/r/Library/Caches/go-build"
GOENV="/Users/r/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOMODCACHE="/Users/r/go/pkg/mod"
GOOS="darwin"
GOPATH="/Users/r/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/r/sdk/go1.20.3"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/r/sdk/go1.20.3/pkg/tool/darwin_arm64"
GOVCS=""
GOVERSION="go1.20.3"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-O2 -g"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-O2 -g"
CGO_FFLAGS="-O2 -g"
CGO_LDFLAGS="-O2 -g"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/j_/rq9ph2cd3h50w468sgv34lwh0000gn/T/go-build3877561209=/tmp/go-build -gno-record-gcc-switches -fno-common"
GOROOT/bin/go version: go version go1.20.3 darwin/arm64
GOROOT/bin/go tool compile -V: compile version go1.20.3
uname -v: Darwin Kernel Version 22.3.0: Mon Jan 30 20:38:37 PST 2023; root:xnu-8792.81.3~2/RELEASE_ARM64_T6000
ProductName:		macOS
ProductVersion:		13.2.1
BuildVersion:		22D68
lldb --version: lldb-1403.0.17.64
Apple Swift version 5.8 (swiftlang-5.8.0.124.2 clang-1403.0.22.11.100)

What did you do?

build this go file:

package main

import "fmt"

func run[T [5]byte | [10]byte](a T) {
	fmt.Println(a[2:4])
}

func main() {
	run([5]byte{})
}

go build test.go

What did you expect to see?

build pass

What did you see instead?

# command-line-arguments
./test.go:6:14: invalid operation: cannot slice a (variable of type T constrained by [5]byte | [10]byte): T has no core type

[zigo101]

A slice operation needs the type parameter constraint has a core type. This is a limitation clearly stated in the current spec. It is unclear whether or not this limitation will be removed in future Go versions.

BTW, the limitation applies to more operations: https://go101.org/generics/777-operations-on-values-of-type-parameter-types.html

[seankhliao]

I think this is #51053 with a different operator.