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

go version go1.6.3 linux/amd64
Also tested with go version go1.6.3 linux/amd64

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH=""
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GO15VENDOREXPERIMENT="1"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0"
CXX="g++"
CGO_ENABLED="1"

What did you do?

mkdir a%2fb
cd a%2fb
mkdir include

Save this Go program as a%2fb/silly.go:

package main

// #cgo CFLAGS: -I"${SRCDIR}/include"
import "C"

func main() {
}

What did you expect to see?

Expected successful compilation/execution of the program.

What did you see instead?

# go run silly.go
package main: /somewhere/a%2Fb/silly.go: malformed #cgo argument: -I/somewhere/a%2Fb/include

This happens for CFLAGS and LDFLAGS on multiple architectures.

Related to #7906, #16455, #11868, which all discuss spaces in paths; there are issues with other valid path characters as well.

This path was not something I created naturally; it was a side-effect of our CI environment (Jenkins) which occasionally uses paths like foo%2Fbar if a Git branch has a name "foo/bar". It also can create paths like foo%2Fbar@2 when a second scratch directory is needed.