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

% go version
go version go1.14.7 darwin/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="/Users/straut/Library/Caches/go-build"
GOENV="/Users/straut/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/straut/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/9m/40r8ccc13c53mcyc85qzjmd400d81r/T/go-build148293827=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

1. In website/content/static/doc/, add an index.html with the following content.

   <!--{
   "Title": "Documentation",
   "Path": "/doc/"
   }-->
   
   <p>This is the index file.</p>
   
2. In website/content/static/internal/gen/gen.go, add the following entry to var files = []string{ ... }:

   var files = []string{
       ...
       "doc/index.html",
       ...
   }
   
3. Open website/content/static/doc/docs.html, and change the header to the following (to avoid a path collision):

   <!--{
   "Title": "Documentation",
   "Path": "/doc/docs",
   "Template": true
   }-->
   
4. cd to the website/ directory

   cd my-repo/website
   
5. Generate the website.

   go generate golang.org/x/website/content/static
   
6. Run the site.

   go run golang.org/x/website/cmd/golangorg -http=:6060
   
7. In a browser, visit the page you just created:

   http://localhost:6060/doc/
   
8. In the console, note three errors ending with the following:

   2020/08/10 09:20:14 http: panic serving [::1]:55541: runtime error: slice bounds out of range [:-5]
   goroutine 1138 [running]:
   net/http.(*conn).serve.func1(0xc0001443c0)
       /usr/local/go/src/net/http/server.go:1800 +0x139
   panic(0x176f6c0, 0xc0006e29c0)
       /usr/local/go/src/runtime/panic.go:975 +0x3e3
   golang.org/x/tools/godoc.(*Presentation).serveFile(0xc0002f8000, 0x1a24aa0, 0xc0001da460, 0xc0003ca400)
       /Users/straut/go/pkg/mod/golang.org/x/tools@v0.0.0-20200617212913-87be026d3888/godoc/server.go:778 +0x87d
   golang.org/x/tools/godoc.(*Presentation).ServeFile(...)
       /Users/straut/go/pkg/mod/golang.org/x/tools@v0.0.0-20200617212913-87be026d3888/godoc/server.go:753
   net/http.HandlerFunc.ServeHTTP(0xc00031a480, 0x1a24aa0, 0xc0001da460, 0xc0003ca400)
       /usr/local/go/src/net/http/server.go:2041 +0x44
   net/http.(*ServeMux).ServeHTTP(0xc000fbc080, 0x1a24aa0, 0xc0001da460, 0xc0003ca400)
       /usr/local/go/src/net/http/server.go:2416 +0x1a5
   golang.org/x/tools/godoc.(*Presentation).ServeHTTP(0xc0002f8000, 0x1a24aa0, 0xc0001da460, 0xc0003ca400)
       /Users/straut/go/pkg/mod/golang.org/x/tools@v0.0.0-20200617212913-87be026d3888/godoc/pres.go:143 +0x4d
   net/http.(*ServeMux).ServeHTTP(0xc000b70540, 0x1a24aa0, 0xc0001da460, 0xc0003ca400)
       /usr/local/go/src/net/http/server.go:2416 +0x1a5
   main.hostEnforcerHandler.ServeHTTP(0x1a18b20, 0xc000b70540, 0x1a24aa0, 0xc0001da460, 0xc0003ca400)
       /Users/straut/Documents/workfiles/go/repo/git/website/cmd/golangorg/handlers.go:45 +0x380
   net/http.(*ServeMux).ServeHTTP(0x1e9e580, 0x1a24aa0, 0xc0001da460, 0xc0003ca400)
       /usr/local/go/src/net/http/server.go:2416 +0x1a5
   net/http.serverHandler.ServeHTTP(0xc0001da2a0, 0x1a24aa0, 0xc0001da460, 0xc0003ca400)
       /usr/local/go/src/net/http/server.go:2836 +0xa3
   net/http.(*conn).serve(0xc0001443c0, 0x1a25e20, 0xc000fbc280)
       /usr/local/go/src/net/http/server.go:1924 +0x86c
   created by net/http.(*Server).Serve
       /usr/local/go/src/net/http/server.go:2962 +0x35c
   

What did you expect to see?

The index page I created loads when I visit the directory in a browser.

What did you see instead?

The panic above.