encoding/json: add cases to TestUnmarshal for fatal syntactic errors

The presence of a syntax error in the input immediately unmarshaling
before unmarshaling into the underlying value.
Otherwise, semantic errors are generally lazily reported and
allow unmarshaling to continue on.

Change-Id: Icf1cfc684e415312d9c8bf739c396ede15299d7d
Reviewed-on: https://go-review.googlesource.com/c/go/+/642295
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/encoding/json/decode_test.go

@@ -1170,6 +1170,23 @@ var unmarshalTests = []struct {
 			N Number `json:",string"`
 		}{"5"},
 	},
+
+	// Verify that syntactic errors are immediately fatal,
+	// while semantic errors are lazily reported
+	// (i.e., allow processing to continue).
+	{
+		CaseName: Name(""),
+		in:       `[1,2,true,4,5}`,
+		ptr:      new([]int),
+		err:      &SyntaxError{msg: "invalid character '}' after array element", Offset: 14},
+	},
+	{
+		CaseName: Name(""),
+		in:       `[1,2,true,4,5]`,
+		ptr:      new([]int),
+		out:      []int{1, 2, 0, 4, 5},
+		err:      &UnmarshalTypeError{Value: "bool", Type: reflect.TypeFor[int](), Offset: 9},
+	},
 }
 
 func TestMarshal(t *testing.T) {