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

$ go version
go version go1.15.2 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/james/.cache/go-build"
GOENV="/home/james/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/james/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/james/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/james/Code/bridge/go.mod"
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-build171979470=/tmp/go-build -gno-record-gcc-switches"

What did you do?

func TestParseDate(t *testing.T) {
	if _, err := mail.ParseDate("Tue, 21 Aug 2018 13:41:49 GMT"); err != nil {
		t.Fail()
	}
}

What did you expect to see?

I expected it to pass, as it did on go 1.13.

What did you see instead?

It fails on newer versions of go (1.14.7 and later, possibly earlier).

The go 1.15 implementation contains the following code block in ParseDate(string) (lines 110-120 of mail/message.go, as of the time of writing):

	// RFC 5322: zone = (FWS ( "+" / "-" ) 4DIGIT) / obs-zone
	// zone length is always 5 chars unless obsolete (obs-zone)
	if ind := strings.IndexAny(p.s, "+-"); ind != -1 && len(p.s) >= ind+5 {
		date = p.s[:ind+5]
		p.s = p.s[ind+5:]
	} else if ind := strings.Index(p.s, "T"); ind != -1 && len(p.s) >= ind+1 {
		// The last letter T of the obsolete time zone is checked when no standard time zone is found.
		// If T is misplaced, the date to parse is garbage.
		date = p.s[:ind+1]
		p.s = p.s[ind+1:]
	}

When executing the above test, we enter the else case, and because the string begins with T for Tue, the variable date ends up with value "T" and p.s ends up with "ue, 21 Aug 2018 13:41:49 GMT".

The function then continues and executes the following loop (lines 124-129 of mail/message.go):

	for _, layout := range dateLayouts {
		t, err := time.Parse(layout, date)
		if err == nil {
			return t, nil
		}
	}

This should succeed, because dateLayouts does indeed contain a matching layout (namely Mon, 2 Jan 2006 15:04:05 MST), but by this point date has been reduced to just T, so it fails.

I feel that the first block of code, which checks the placement of T, shouldn't break the date string, as in the test case provided above.

Go 1.13 did not contain the above mentioned block of code that reduces date down to "T" and thus successfully parses the provided date string.