Unexported map value causes go-cmp to panic

[btmura]

Input:

func TestBreakCmp(t *testing.T) {

type unexported int
type Foo struct {
	m map[string]unexported
}

f1 := &Foo{m: map[string]unexported{"foo": 1, "bar": 2}} // One key will work. Multiple causes panic.
f2 := &Foo{m: map[string]unexported{"bar": 3, "baz": 4}}

// Passes!
cmp.Diff(f1, f2, cmp.Comparer(func(_, _ Foo) bool {
	return true
}))

// Panics!
cmp.Diff(f1, f2, cmp.Comparer(func(_, _ Foo) bool {
	return false
}))

}

Output:

panic: reflect.Value.Interface: cannot return value obtained from unexported field or method [recovered]
panic: ,reflect.Value.Interface: cannot return value obtained from unexported field or method

goroutine 6 [running]:
testing.tRunner.func1(0xc0420a40f0)
C:/Go/src/testing/testing.go:711 +0x2d9
panic,(0x578e40, 0x5d5660)
C:/Go/src/runtime/panic.go:491 +0x291
,reflect.valueInterface(0x578e40, 0xc0421065b0, 0xb8, ,0x1, 0x4, 0x2)
C:/Go/src/reflect/value.go:936, +0x1e4
reflect.Value.Interface(0x578e40, 0xc0421065b0,, 0xb8, 0xc0420fe3e0, 0xc042136000)
C:/Go/src/reflect/value.go,:925 +0x4b
github.com/google/go-cmp/cmp/internal/value.SortKeys(0xc042102060,, 0x2, 0x2, 0xc042102060, 0x2, ,0x2)
D:/work/go/src/github.com/google/go-cmp/cmp/internal/value/sort.go:27 +0x16d
,github.com/google/go-cmp/cmp/internal/value.formatAny(0x587e20, 0xc0420026d8, 0x1b5,, 0x1010100, 0x0, 0x25, 0x688ce0)
, D:/work/go/src/github.com/google/go-cmp/cmp/internal/value/format.go:132 +0x1a38
github.com/google/go-cmp/cmp/internal/value.formatAny,(0x590c40, 0xc0420026c0, 0x199, 0xc001010101, ,0x0, 0xc042100012, 0x9)
D:/work/go/src/github.com/google/go-cmp/cmp/internal/value/format.go:,153 +0xea0
github.com/google/go-cmp/cmp/internal/value.Format(0x590c40, 0xc0420026c0, 0x199, 0x4bf901, 0xc042100012, 0x9)
D:/work/go/src/github.com/google/go-cmp/cmp/internal/value/format.go:30 +0x6d
github.com/google/go-cmp/cmp.(*defaultReporter).Report(0xc042038c40, 0x590c40,, 0xc042002680, 0x199, 0x590c40, 0xc0420026c0, 0x199, ,0x590c00, 0xc0420028c0, 0x2, ...)
D:/work/go/src/github.com/google/go-cmp/cmp/reporter.go:,34 +0xf7
github.com/google/go-cmp/cmp.(*state).report(0xc042082310, 0x583f00,, 0x590c40, 0xc042002680, 0x199, 0x590c40, ,0xc0420026c0, 0x199)
D:/work/go/src/github.com/google/go-cmp/cmp/compare.go:492, +0xce
github.com/google/go-cmp/cmp.(*comparer).apply(0xc042070630, 0xc042082310,, 0x590c40, 0xc042002680, 0x199, 0x590c40, 0xc0420026c0,, 0x199, 0xc0420026c0)
D:/work/go/src/github.com/google/go-cmp/cmp/options.go:315, +0x103
github.com/google/go-cmp/cmp.(*state).tryOptions(0xc042082310, 0x590c40, ,0xc042002680, 0x199, 0x590c40, 0xc0420026c0, 0x199,, 0x688ce0, 0x590c40, 0x199)
D:/work/go/src/github.com/google/go-cmp/cmp/compare.go:307 +0x154
github.com/google/go-cmp/cmp.(*state).compareAny,(0xc042082310, 0x590c40, 0xc042002680, 0x199, 0x590c40,, 0xc0420026c0, 0x199)
D:/work/go/src/github.com/google/go-cmp/cmp/compare.go:202, +0x343
github.com/google/go-cmp/cmp.(*state).compareAny(0xc042082310, 0x5857e0, ,0xc042002680, 0x16, 0x5857e0, 0xc0420026c0, 0x16)
D:/work/go/src/github.com/google/go-cmp/cmp/compare.go:244 +0x1540
github.com/google/go-cmp/cmp.Equal(0x5857e0, 0xc042002680, 0x5857e0, 0xc0420026c0, 0xc042002880, 0x2, 0x2, 0x590c40)
D:/work/go/src/github.com/google/go-cmp/cmp/compare.go:86 +0xf8
github.com/google/go-cmp/cmp.Diff(0x5857e0, 0xc042002680, 0x5857e0, 0xc0420026c0, 0xc042002860, 0x2, 0x2, 0xc042093f98, 0x4f6b55)
D:/work/go/src/github.com/google/go-cmp/cmp/compare.go,:100 +0x13a

[dsnet]

Thanks for the great succinct reproduction. This was an interesting failure, but the fix was easy.