testing: Helper does not appear to work in presence of panic

[myitcv]

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

$ go version
go version go1.12.1 linux/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
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/myitcv/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="$WORK/gopath"
GOPROXY=""
GORACE=""
GOROOT="/home/myitcv/gos"
GOTMPDIR=""
GOTOOLDIR="/home/myitcv/gos/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=$WORK/tmp/go-build690459405=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Came to light in https://go-review.googlesource.com/c/go/+/168886

Ran testscript on the following:

! go test
stdout 'main_test.go:6: expected panic'
stdout 'main_test.go:7: expected S, got string'

-- main_test.go --
package main

import "testing"

func Test(t *testing.T) {
        blah(t, func() {})
        blah2(t, func() { panic("oh dear") })
}

func blah(t *testing.T, f func()) {
        t.Helper()

        defer func() {
                t.Helper()

                err := recover()
                if err == nil {
                        t.Errorf("expected panic")
                }
        }()
        f()
}

func blah2(t *testing.T, f func()) {
        t.Helper()

        defer func() {
                t.Helper()
                err := recover()
                if _, ok := err.(S); !ok {
                        t.Errorf("expected S, got %T", err)
                }
        }()
        f()
}

type S struct{}

What did you expect to see?

A passing test

What did you see instead?

> ! go test
[stdout]
--- FAIL: Test (0.00s)
    main_test.go:6: expected panic
    panic.go:522: expected S, got string
FAIL
exit status 1
FAIL    _$WORK  0.002s

[exit status 1]
> stdout 'main_test.go:6: expected panic'
> stdout 'main_test.go:7: expected S, got string'
FAIL: /tmp/testscript728881261/0/script.txt:5: no match for `main_test.go:7: expected S, got string` found in stdout
error running t_helper_panic.txt in /tmp/testscript728881261/0

FYI @neelance

[josharian]

@cespare

[gopherbot]

Change https://golang.org/cl/345909 mentions this issue: testing: skip panics when picking the line number for decoration

[cespare]

Sure, this is easy to change. I sent a PR. It's surprising to me that nobody else has seemingly noticed or complained.