Repository: go
Issue: cmd/compile: internal compiler error: Type.Elem UNSAFEPTR
Labels: NeedsInvestigation,compiler/runtime

<!-- Please answer these questions before submitting your issue. Thanks! -->

### What version of Go are you using (`go version`)?

<pre>
$ go version
go version devel +4091cf972a Sun Mar 31 23:35:35 2019 +0000 linux/amd64
</pre>

### Does this issue reproduce with the latest release?

reproducible only on tip

### What operating system and processor architecture are you using (`go env`)?

<details><summary><code>go env</code> Output</summary><br><pre>
$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/travis/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/travis/gopath"
GOPROXY=""
GORACE=""
GOROOT="/home/travis/.gimme/versions/go"
GOTMPDIR=""
GOTOOLDIR="/home/travis/.gimme/versions/go/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=/tmp/go-build848806544=/tmp/go-build -gno-record-gcc-switches"
</pre></details>

### What did you do?

<!--
If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.
-->

I have a pretty old package with some hacks for optimization.
I use some of its features in my web framework.
In travis, I check if the framework pass the tests on tip.
One of the steps in travis script is the `go get -v`.
When go get tries to precompile that old package, it crashes.

So, one of the files looks like this:

```
package runtimer

import (
	"unsafe" // #nosec
)

func PtrToString(ptr unsafe.Pointer) string {
	return *(*string)(ptr)
}

func PtrToStringPtr(ptr unsafe.Pointer) *string {
	return (*string)(ptr)
}

func PtrPtrToStringPtr(ptr *unsafe.Pointer) *string {
	return (*string)(*ptr) // <- compiler complains about this line
}
```

Then I've commented out the last func.

Then the file looks like this:

```
package runtimer

import (
	"unsafe" // #nosec
)

func PtrToString(ptr unsafe.Pointer) string {
	return *(*string)(ptr)
}

func PtrToStringPtr(ptr unsafe.Pointer) *string {
	return (*string)(ptr) // <- and then it complains about this line
}
/*
func PtrPtrToStringPtr(ptr *unsafe.Pointer) *string {
	return (*string)(*ptr)
}
*/
```

### What did you expect to see?

`unsafe.Pointer` works as it work two years ago

### What did you see instead?

```
# github.com/gramework/runtimer
../runtimer/utils.go:16:2: internal compiler error: Type.Elem UNSAFEPTR
goroutine 34 [running]:
runtime/debug.Stack(0x1039ae0, 0xc00000e018, 0x0)
	/home/travis/.gimme/versions/go/src/runtime/debug/stack.go:24 +0x9d
cmd/compile/internal/gc.Fatalf(0xe8ef7d, 0xc, 0xc0007c7550, 0x1, 0x1)
	/home/travis/.gimme/versions/go/src/cmd/compile/internal/gc/subr.go:190 +0x292
cmd/compile/internal/types.(*Type).Elem(0xc000061e60, 0xc000538460)
	/home/travis/.gimme/versions/go/src/cmd/compile/internal/types/type.go:801 +0xff
cmd/compile/internal/ssa.(*Func).computeZeroMap(0xc000804000, 0xe12f01)
	/home/travis/.gimme/versions/go/src/cmd/compile/internal/ssa/writebarrier.go:391 +0x101
cmd/compile/internal/ssa.writebarrier(0xc000804000)
	/home/travis/.gimme/versions/go/src/cmd/compile/internal/ssa/writebarrier.go:80 +0x6b
cmd/compile/internal/ssa.Compile(0xc000804000)
	/home/travis/.gimme/versions/go/src/cmd/compile/internal/ssa/compile.go:90 +0x476
cmd/compile/internal/gc.buildssa(0xc0003c9b80, 0x0, 0x0)
	/home/travis/.gimme/versions/go/src/cmd/compile/internal/gc/ssa.go:288 +0xbf3
cmd/compile/internal/gc.compileSSA(0xc0003c9b80, 0x0)
	/home/travis/.gimme/versions/go/src/cmd/compile/internal/gc/pgen.go:297 +0x4d
cmd/compile/internal/gc.compileFunctions.func2(0xc000404de0, 0xc000407200, 0x0)
	/home/travis/.gimme/versions/go/src/cmd/compile/internal/gc/pgen.go:362 +0x49
created by cmd/compile/internal/gc.compileFunctions
	/home/travis/.gimme/versions/go/src/cmd/compile/internal/gc/pgen.go:360 +0x128
```