IntSliceFlag and friends don't work correctly with destination (+ behavior quirks relating to value)

[joeycumines]

My urfave/cli version is

v2.8.1

Checklist

• Are you running the latest v2 release? The list of releases is here.
• Did you check the manual for your release? The v2 manual is here
• Did you perform a search about this problem? Here's the Github guide about searching.

Dependency Management

• My project is using go modules.

Describe the bug

I've noticed a some issues / inconsistencies, while implementing #1409. I'm creating this issue just to make it clear what they are.

• It appears Destination fields were added to the IntSliceFlag, Int64SliceFlag, and Float64SliceFlag structs, without actually implementing the necessary behavior
• The StringSliceFlag.Value (default value) field may be modified by values passed in by environment variables
• It doesn't seem like a problem, but the Value field seems like it should never be modified (e.g. don't initialise it if it's nil / use a local variable instead)

Past issues/PRs that may be relevant: #1235, #981, #1178

Currently open issues that may be relevant: #1224, #1143

To reproduce

N/A, but take a look at #1409, particularly the existing test I modified (for the second issue)

Observed behavior

See above

Expected behavior

The value / destination support of IntSliceFlag etc should be equivalent to StringSliceFlag. That said, there's a bit of a question around handling the (set) empty string env var case, however. See the note on my PR.

Run go version and paste its output here

go version go1.18.2 linux/amd64

Run go env and paste its output here

GO111MODULE="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/joey/.cache/go-build"
GOENV="/home/joey/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/joey/under/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/joey/under/go"
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="go1.18.2"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/f/dev/urfave-cli/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-build1554591512=/tmp/go-build -gno-record-gcc-switches"