Add tests for unrecognized reflect values.

In order to help future proof itself, spew handles unrecognized reflect
values by passing them on to the standard fmt library.  Since spew handles
all current reflect values in the language, this condition has to be
manually tested with a bit of hackery by using unsafe to change the kind
to a nonexistent value.

As of this commit, there is now 100% test coverage.

spew/internal_test.go

@@ -26,6 +26,7 @@ import (
 	"bytes"
 	"reflect"
 	"testing"
+	"unsafe"
 )
 
 // dummyFmtState implements a fake fmt.State to use for testing invalid
@@ -79,3 +80,78 @@ func TestInvalidReflectValue(t *testing.T) {
 		t.Errorf("InvalidReflectValue #%d got: %s want: %s", i, s, want)
 	}
 }
+
+// flagRO, flagKindShift and flagKindWidth indicate various bit flags that the
+// reflect package uses internally to track kind and state information.
+const flagRO = 1 << 0
+const flagKindShift = 4
+const flagKindWidth = 5
+
+// changeKind uses unsafe to intentionally change the kind of a reflect.Value to
+// the maximum kind value which does not exist.  This is needed to test the
+// fallback code which punts to the standard fmt library for new types that
+// might get added to the language.
+func changeKind(v *reflect.Value, readOnly bool) {
+	rvf := (*uintptr)(unsafe.Pointer(uintptr(unsafe.Pointer(v)) +
+		unsafe.Offsetof(reflectValue.flag)))
+	*rvf = *rvf | ((1<<flagKindWidth - 1) << flagKindShift)
+	if readOnly {
+		*rvf |= flagRO
+	} else {
+		*rvf &= ^uintptr(flagRO)
+	}
+}
+
+// TestAddedReflectValue tests functionaly of the dump and formatter code which
+// falls back to the standard fmt library for new types that might get added to
+// the language.
+func TestAddedReflectValue(t *testing.T) {
+	i := 1
+
+	// Dump using a reflect.Value that is exported.
+	v := reflect.ValueOf(int8(5))
+	changeKind(&v, false)
+	buf := new(bytes.Buffer)
+	d := dumpState{w: buf, cs: &Config}
+	d.dump(v)
+	s := buf.String()
+	want := "(int8) 5"
+	if s != want {
+		t.Errorf("TestAddedReflectValue #%d\n got: %s want: %s", i, s, want)
+	}
+	i++
+
+	// Dump using a reflect.Value that is not exported.
+	changeKind(&v, true)
+	buf.Reset()
+	d.dump(v)
+	s = buf.String()
+	want = "(int8) <int8 Value>"
+	if s != want {
+		t.Errorf("TestAddedReflectValue #%d\n got: %s want: %s", i, s, want)
+	}
+	i++
+
+	// Formatter using a reflect.Value that is exported.
+	changeKind(&v, false)
+	buf2 := new(dummyFmtState)
+	f := formatState{value: v, cs: &Config, fs: buf2}
+	f.format(v)
+	s = buf2.String()
+	want = "5"
+	if s != want {
+		t.Errorf("TestAddedReflectValue #%d got: %s want: %s", i, s, want)
+	}
+	i++
+
+	// Formatter using a reflect.Value that is not exported.
+	changeKind(&v, true)
+	buf2.Reset()
+	f = formatState{value: v, cs: &Config, fs: buf2}
+	f.format(v)
+	s = buf2.String()
+	want = "<int8 Value>"
+	if s != want {
+		t.Errorf("TestAddedReflectValue #%d got: %s want: %s", i, s, want)
+	}
+}