go/doc: Examples panics on a file containing a function without a body

[bmizerany]

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

go version go1.15.5 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

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

go env Output

GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/bmizerany/Library/Caches/go-build"
GOENV="/Users/bmizerany/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/bmizerany/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/bmizerany/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/c0/8wpgkv1d47n5lxdmw1q79nzm0000gn/T/go-build255996969=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Wrote this example:

package example_test

func foo(int)

func Example() {
        foo(1)
}

Ran go test

What did you expect to see?

go test succeed or error with helpful message.

What did you see instead?

A panic:

$ ls
example_test.go

$ cat example_test.go 
package example_test

func foo(int)

func Example() {
	foo(1)
}

$ go test
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x1121b37]

goroutine 1 [running]:
go/ast.Walk(0x1704080, 0xc00029c930, 0x17089e0, 0x0)
	/usr/local/go/src/go/ast/walk.go:224 +0x18b7
go/ast.Inspect(...)
	/usr/local/go/src/go/ast/walk.go:385
go/doc.playExample(0xc0002b4580, 0xc00029c810, 0x0)
	/usr/local/go/src/go/doc/example.go:240 +0x465
go/doc.Examples(0xc0001652b0, 0x1, 0x1, 0x9, 0x0, 0x4)
	/usr/local/go/src/go/doc/example.go:89 +0x5ec
cmd/go/internal/load.(*testFuncs).load(0xc0002d6150, 0xc000428040, 0x32, 0x163809d, 0x6, 0xc0002d61aa, 0xc0002d61ab, 0x0, 0x0)
	/usr/local/go/src/cmd/go/internal/load/test.go:585 +0x10c
cmd/go/internal/load.loadTestFuncs(0xc000080d80, 0x5, 0x5, 0x0)
	/usr/local/go/src/cmd/go/internal/load/test.go:472 +0x29e
cmd/go/internal/load.TestPackagesAndErrors(0xc000080d80, 0x0, 0xc000081680, 0xc000080d80, 0xc000081200)
	/usr/local/go/src/cmd/go/internal/load/test.go:271 +0x1cb6
cmd/go/internal/load.TestPackagesFor(0xc000080d80, 0x0, 0x1448576, 0x15a8100, 0xc00009a540, 0x10fbf1b, 0xc0000c4760)
	/usr/local/go/src/cmd/go/internal/load/test.go:46 +0x39
cmd/go/internal/test.builderTest(0xc0000bf7c0, 0xc000080d80, 0x0, 0x0, 0x0, 0x1703780, 0xc000152030)
	/usr/local/go/src/cmd/go/internal/test/test.go:838 +0x6e
cmd/go/internal/test.runTest(0x19ce8e0, 0xc0000a8030, 0x0, 0x0)
	/usr/local/go/src/cmd/go/internal/test/test.go:748 +0x4d0
main.main()
	/usr/local/go/src/cmd/go/main.go:190 +0x58d

NOTE: The func foo has no body. When I add the body like func foo(int) {} it does not panic.

[qbradq]

I looked into this a bit. parser.ParseFile and friends do not generate an error for the function with no body but the compiler does. This results in tools like go test and gopls to panic in this way. Would it be appropriate to try to add this as an error condition to the parser or should I just patch src/go/doc/example.go ?

[ianlancetaylor]

A function with no body is valid Go. It is expected to be provided by assembler code or C code or via go:linkname or via some other mechanism. See https://golang.org/ref/spec#Function_declarations .

I haven't looked at this and I'm not sure where it is going wrong, but it would not be correct to modify the parser to reject functions with no bodies.

[qbradq]

Forgive me if this is not constructive. When I try to run an application with a function with no body I get a compilation error. Go Playground Example. This is why I assumed it was not valid. Does that imply an error in the compiler?

[ianlancetaylor]

Function bodies may only be omitted in packages that contain assembler code or use cgo or use go:linkname.

[gopherbot]

Change https://golang.org/cl/275516 mentions this issue: go/doc: avoid panic on references functions with no body