decoder: don't panic when path crosses an unexported pointer field

decoder.go

@@ -279,6 +279,11 @@ func (d *Decoder) decode(v reflect.Value, path string, parts []pathPart, values
 	for _, name := range parts[0].path {
 		if v.Type().Kind() == reflect.Ptr {
 			if v.IsNil() {
+				// Allocating into an unexported pointer panics; the path
+				// targets a field we can't set anyway, so bail early.
+				if !v.CanSet() {
+					return nil
+				}
 				v.Set(reflect.New(v.Type().Elem()))
 			}
 			v = v.Elem()
@@ -288,7 +293,7 @@ func (d *Decoder) decode(v reflect.Value, path string, parts []pathPart, values
 		if v.Type().Kind() == reflect.Struct {
 			for i := 0; i < v.NumField(); i++ {
 				field := v.Field(i)
-				if field.Type().Kind() == reflect.Ptr && field.IsNil() && v.Type().Field(i).Anonymous {
+				if field.Type().Kind() == reflect.Ptr && field.IsNil() && v.Type().Field(i).Anonymous && field.CanSet() {
 					field.Set(reflect.New(field.Type().Elem()))
 				}
 			}

decoder_test.go

@@ -717,6 +717,41 @@ func TestUnexportedField(t *testing.T) {
 	}
 }
 
+// Regression: walking through an unexported pointer field used to panic
+// with "reflect.Value.Set using value obtained using unexported field"
+// because Decoder.decode tried to allocate the pointer before realizing
+// the target was unreachable.
+type s6WithNestedUnexported struct {
+	A string
+	c *struct {
+		X string
+	}
+}
+
+func TestUnexportedPointerFieldNestedPathDoesNotPanic(t *testing.T) {
+	defer func() {
+		if r := recover(); r != nil {
+			t.Fatalf("Decode panicked on unexported pointer path: %v", r)
+		}
+	}()
+	dec := NewDecoder()
+	dec.IgnoreUnknownKeys(true)
+	s := &s6WithNestedUnexported{}
+	err := dec.Decode(s, map[string][]string{
+		"A":   {"hi"},
+		"c.X": {"should not panic"},
+	})
+	if err != nil {
+		t.Fatalf("Decode returned an error: %v", err)
+	}
+	if s.A != "hi" {
+		t.Errorf("A: got %q, want %q", s.A, "hi")
+	}
+	if s.c != nil {
+		t.Errorf("c expected to be left nil, got %+v", s.c)
+	}
+}
+
 // ----------------------------------------------------------------------------
 
 type S7 struct {