go/types: panic: invalid package name "_"

[josharian]

package p
	
import "_"

Check with go/types, using Config:

	conf := types.Config{
		Error:    func(err error) {}, // important for reproduction
	}

Result is panic while executing conf.Check:

panic: invalid package name _ [recovered]
	panic: invalid package name _ [recovered]
	panic: invalid package name _

goroutine 18 [running]:
testing.tRunner.func1(0xc4200f80f0)
	/Users/josh/go/tip/src/testing/testing.go:712 +0x256
panic(0x12914e0, 0x1334390)
	/Users/josh/go/tip/src/runtime/panic.go:489 +0x259
go/types.(*Checker).handleBailout(0xc4200fa000, 0xc420071d18)
	/Users/josh/go/tip/src/go/types/check.go:225 +0xa4
panic(0x12914e0, 0x1334390)
	/Users/josh/go/tip/src/runtime/panic.go:489 +0x259
go/types.NewPackage(0xc420088391, 0x1, 0xc420088391, 0x1, 0xffffffffffffffff)
	/Users/josh/go/tip/src/go/types/package.go:27 +0x439
go/types.(*Checker).importPackage(0xc4200fa000, 0x15, 0xc420088391, 0x1, 0x12f52f4, 0x1, 0x9)
	/Users/josh/go/tip/src/go/types/resolver.go:172 +0x460
go/types.(*Checker).collectObjects(0xc4200fa000)
	/Users/josh/go/tip/src/go/types/resolver.go:243 +0x1c04
go/types.(*Checker).checkFiles(0xc4200fa000, 0xc420046e30, 0x1, 0x1, 0x0, 0x0)
	/Users/josh/go/tip/src/go/types/check.go:237 +0xa1
go/types.(*Checker).Files(0xc4200fa000, 0xc420046e30, 0x1, 0x1, 0xc420080c90, 0x0)
	/Users/josh/go/tip/src/go/types/check.go:230 +0x49
go/types.(*Config).Check(0xc42008ab80, 0x12f531e, 0x1, 0xc42008ab00, 0xc420046e30, 0x1, 0x1, 0xc4200955e0, 0x0, 0x1, ...)
	/Users/josh/go/tip/src/go/types/api.go:351 +0x1b3
go/types_test.TestIssue16902(0xc4200f80f0)
	/Users/josh/go/tip/src/go/types/sizes_test.go:102 +0x2ad
testing.tRunner(0xc4200f80f0, 0x130bc80)
	/Users/josh/go/tip/src/testing/testing.go:747 +0xd0
created by testing.(*T).Run
	/Users/josh/go/tip/src/testing/testing.go:789 +0x2de
exit status 2

Discovered with go-fuzz.

@griesemer @mvdan

[gopherbot]

CL https://golang.org/cl/42852 mentions this issue.