flag: Error wrapping is missing from the flag package

[VoyTechnology]

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

$ go version
go version go1.19.1 darwin/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="/Users/voy/.go/bin"
GOCACHE="/Users/voy/Library/Caches/go-build"
GOENV="/Users/voy/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/voy/.go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/voy/.go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.19.1/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.19.1/libexec/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.19.1"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/voy/.go/src/REDACTED/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 -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/var/folders/sj/5vkqw5217xn221ywb9d95bzc0000gn/T/go-build832936302=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

https://go.dev/play/p/SVo8u-vFr3T

What did you expect to see?

No output as the underlying error is the same.

What did you see instead?

The errors do not match as the error returned from Value.Set is not wrapped using %w in (*FlagSet).parseOne

go/src/flag/flag.go

Line 1088 in 36760ca

return false, f.failf("invalid value %q for flag -%s: %v", value, name, err)

[ianlancetaylor]

That's not how errors.Is works. errors.Is doesn't report whether an error prints the same. It reports whether the error is the same. errors.Is will never report true when called with an error returned from the flag package and an error that you've defined yourself. See the docs for errors.New: "Each call to New returns a distinct error value even if the text is identical."

[VoyTechnology]

Hi ianlancetaylor, thank you for your response. I am aware that the errors.Is does not compare the error string representation. The problem is that the error returned by Set is not wrapped in the error chain by the flag package. As shown in

go/src/flag/flag.go

Line 1088 in 36760ca

return false, f.failf("invalid value %q for flag -%s: %v", value, name, err)

I would expect that the error was wrapped by using %w rather than converted into a string using %v.

[D1CED]

Consider opening a new issue requesting that the flag package shall return a new kind of error in case a flag.Value.Set returns an error so that one is able to inspect it.

[ianlancetaylor]

Today I can't see any reason for the flag package to use %w, because the wrapped error is not exported.

[seankhliao]

There are cases like flag.Func where the user can return an error that's later surfaced through flag.Parse.
Wrapping those errors would allow users to test for their own validation errors.

[tsuna]

Isn't the code sample posted by OP on the Go Playground another example of a user-controller error type returned eventually through flag.Parse()?