time: error handling for time.Month.String() and time.Weekday.String() is inconsistent

[jaseemabid]

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

go version go1.10 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/j/.cache/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/j/go"
GORACE=""
GOROOT="/usr/lib/go"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
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-build714199702=/tmp/go-build -gno-record-gcc-switches"

What did you do?

When converting a month out of range to String, the function returns a debug string, but panics for Weekday.

Code sample at https://play.golang.org/p/3U4gRY5kRzc

What did you expect to see?

Both should either panic, or return error string.

What did you see instead?

time.Month.String() returned debug string, time.Weekday.String() paniced.

[agnivade]

Indeed.

Month.String() checks for range, and if outside it returns a stringified representation of it.

// String returns the English name of the month ("January", "February", ...).
func (m Month) String() string {
	if January <= m && m <= December {
		return months[m-1]
	}
	buf := make([]byte, 20)
	n := fmtInt(buf, uint64(m))
	return "%!Month(" + string(buf[n:]) + ")"
}

Whereas Weekday.String() does not.

// String returns the English name of the day ("Sunday", "Monday", ...).
func (d Weekday) String() string { return days[d] }

We should probably fix to not panic on (Weekday).String(), or there might be a reason for this. Not sure. In any case, I think this might be on the verge of breaking Go1 compat. NeedsDecision.

/cc @rsc, @ianlancetaylor

[ianlancetaylor]

We should fix Weekday.String as we fixed Month.String for #17720.

[gopherbot]

Change https://golang.org/cl/106535 mentions this issue: time: don't panic when stringifying Weekday