Fix unmarshaling of nested non-exported struct (#917)

Fixes #915

Author: pelletier

Diff for: unmarshaler.go

@@ -1097,9 +1097,9 @@ func (d *decoder) handleKeyValuePart(key unstable.Iterator, value *unstable.Node
 
 		f := fieldByIndex(v, path)
 
-		if !f.CanSet() {
-			// If the field is not settable, need to take a slower path and make a copy of
-			// the struct itself to a new location.
+		if !f.CanAddr() {
+			// If the field is not addressable, need to take a slower path and
+			// make a copy of the struct itself to a new location.
 			nvp := reflect.New(v.Type())
 			nvp.Elem().Set(v)
 			v = nvp.Elem()

Diff for: unmarshaler_test.go

@@ -2801,6 +2801,28 @@ res = [
 	}
 }
 
+func TestIssue915(t *testing.T) {
+	type blah struct {
+		A string `toml:"a"`
+	}
+
+	type config struct {
+		Fizz string `toml:"fizz"`
+		blah `toml:"blah"`
+	}
+
+	b := []byte(`
+fizz = "abc"
+blah.a = "def"`)
+	var cfg config
+	err := toml.Unmarshal(b, &cfg)
+	require.NoError(t, err)
+
+	require.Equal(t, "abc", cfg.Fizz)
+	require.Equal(t, "def", cfg.blah.A)
+	require.Equal(t, "def", cfg.A)
+}
+
 func TestUnmarshalDecodeErrors(t *testing.T) {
 	examples := []struct {
 		desc string