Please answer these questions before submitting your issue. Thanks!

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

go version go1.9 linux/amd64

Does this issue reproduce with the latest release?

Yes

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"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build881373539=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

I would like to build a go package that includes fortran source code files, similar to https://github.com/golang/go/tree/master/misc/cgo/fortran. The important difference from this example is that the package that I would like to compile includes fortran modules, for example:

package.go // depends on code in a_subroutine.f90
a_subroutine.f90 // depends on code in b_module.f90
b_module.90 

I would expect to be able to compile this with go build.

What did you expect to see?

successful compilation.

What did you see instead?

# {...}
<command-line>:12:6:

Fatal Error: Can't open module file ‘b_module.mod’ for reading at (1): No such file or directory

I think the problem might be that the order that fortran files are fed into the compiler is significant, and the go tool does not appear to account for this.

Potential fixes for this problem:

1. renaming all of the fortran files in the directory so that their alphabetical order is also their required compilation order. While this (usually?) works, it appears to depend on undocumented behavior.
2. resolving the fortran dependencies and figuring out the file order within the go tool. This seems unlikely to happen.
3. Allowing the specification of files to be compiled (and their order) a la cgo: Nowhere to specify C/C++ source files #12953.
4. Others?