encoding/json: Only allow string option for valid types

The "string" option only applies for strings, floats, integers, and
booleans as per the documentation. So when decoding ignore the "string"
option if the value is not of one of the types mentioned. This matches
the Marshal step which also ignores the "string" option for invalid
types.

Fixes #9812

Change-Id: I0fb2b43d0668bc0e2985886d989abbf2252070e2
Reviewed-on: https://go-review.googlesource.com/10183Reviewed-by: Russ Cox <rsc@golang.org>

src/encoding/json/decode_test.go

@@ -1393,3 +1393,27 @@ func TestInvalidUnmarshal(t *testing.T) {
 		}
 	}
 }
+
+// Test that string option is ignored for invalid types.
+// Issue 9812.
+func TestInvalidStringOption(t *testing.T) {
+	num := 0
+	item := struct {
+		T time.Time         `json:",string"`
+		M map[string]string `json:",string"`
+		S []string          `json:",string"`
+		A [1]string         `json:",string"`
+		I interface{}       `json:",string"`
+		P *int              `json:",string"`
+	}{M: make(map[string]string), S: make([]string, 0), I: num, P: &num}
+
+	data, err := Marshal(item)
+	if err != nil {
+		t.Fatalf("Marshal: %v", err)
+	}
+
+	err = Unmarshal(data, &item)
+	if err != nil {
+		t.Fatalf("Unmarshal: %v", err)
+	}
+}

src/encoding/json/encode.go

@@ -1043,6 +1043,19 @@ func typeFields(t reflect.Type) []field {
 					ft = ft.Elem()
 				}
 
+				// Only strings, floats, integers, and booleans can be quoted.
+				quoted := false
+				if opts.Contains("string") {
+					switch ft.Kind() {
+					case reflect.Bool,
+						reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
+						reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64,
+						reflect.Float32, reflect.Float64,
+						reflect.String:
+						quoted = true
+					}
+				}
+
 				// Record found field and index sequence.
 				if name != "" || !sf.Anonymous || ft.Kind() != reflect.Struct {
 					tagged := name != ""
@@ -1055,7 +1068,7 @@ func typeFields(t reflect.Type) []field {
 						index:     index,
 						typ:       ft,
 						omitEmpty: opts.Contains("omitempty"),
-						quoted:    opts.Contains("string"),
+						quoted:    quoted,
 					}))
 					if count[f.typ] > 1 {
 						// If there were multiple instances, add a second,