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

$ go version
go version go1.18.2 linux/amd64

Does this issue reproduce with the latest release?

yes, I use the latst stable release

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

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/gaissmai/.cache/go-build"
GOENV="/home/gaissmai/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/gaissmai/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/gaissmai/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/snap/go/9760"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/snap/go/9760/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.18.2"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/gaissmai/project/tests/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-build1518686161=/tmp/go-build -gno-record-gcc-switches"
GOROOT/bin/go version: go version go1.18.2 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.18.2
uname -sr: Linux 5.13.0-44-generic
Distributor ID:	Ubuntu
Description:	Ubuntu 21.10
Release:	21.10
Codename:	impish
gdb --version: GNU gdb (Ubuntu 11.1-0ubuntu2) 11.1

What did you do?

https://go.dev/play/p/2KQHE36RXEa

The type changes after applying Prefix.Masked() from 4in6 to 6, but Masked() should only mask the hostbits and not producing nonsense.

What did you expect to see?

You should change the limitations for type netip.Prefix to:
// Prefix is an IP address prefix (CIDR) representing an IP network.
//
// The first Bits() of Addr() are specified. The remaining bits match any address.
// The range of Bits() is [0,32] for IPv4 or [0,128] for IPv6.

to

// Prefix is an IP address prefix (CIDR) representing an IP network.
//
// The first Bits() of Addr() are specified. The remaining bits match any address.
// The range of Bits() is [0,32] for IPv4 or [0,128] for IPv6 and [96,128] for IP4In6

and netip.ParsePrefix should return an error

What did you see instead?

after applying netip.Prefix.Masked() the type of the IP address changed from IP4In6 to IP6 and the result is nonsense, because more than the hostbits are masked.

prefix: ::ffff:10.5.6.7/88, addr: ::ffff:10.5.6.7, is4in6: true
masked: ::ff00:0:0/88, addr: ::ff00:0:0, is4in6: false