cmd/vet: spurious "context leak" due to imprecise 'defer' control-flow analysis in lostcancel

Please answer these questions before submitting your issue. Thanks!

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

go version go1.10 windows/amd64

Does this issue reproduce with the latest release?

yes, tested on go1.10.2 as well

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

set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\micro\AppData\Local\go-build
set GOEXE=.exe
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOOS=windows
set GOPATH=F:\goenv\
set GORACE=
set GOROOT=F:\go
set GOTMPDIR=
set GOTOOLDIR=F:\go\pkg\tool\windows_amd64
set GCCGO=gccgo
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\micro\AppData\Local\Temp\go-build748724598=/tmp/go-build -gno-record-gcc-switches

What did you do?

run go vet on https://play.golang.org/p/X_vRJbYf7rO

What did you expect to see?

no errors

What did you see instead?

issue.go:12: the cancel function is not used on all paths (possible context leak)
issue.go:14: this return statement may be reached without using the cancel var defined on line 12