Unmarshal (but not Decode) panics instead of returning an error when a type's
UnmarshalJSON returns an error.

For example, add this to decode_test.go:

// Time3339 is a time.Time which encodes to and from JSON                               
                                                              
// as an RFC 3339 time in UTC.                                                          
                                                              
type Time3339 time.Time

func (t *Time3339) UnmarshalJSON(b []byte) error {
        if len(b) < 2 || b[0] != '"' || b[len(b)-1] != '"' {
        return fmt.Errorf("types: failed to unmarshal non-string value %q as an RFC 3339 time")
        }
        tm, err := time.Parse(time.RFC3339, string(b[1:len(b)-1]))
    if err != nil {
                return err
        }
    *t = Time3339(tm)
        return nil
}

func TestUnmarshalJSONLiteralError(t *testing.T) {
        var t3 Time3339
        err := Unmarshal([]byte(`"0000-00-00T00:00:00Z"`), &t3)
        if err == nil {
                t.Fatalf("expected error; got time %v", time.Time(t3))
        }
    if !strings.Contains(err.Error(), "range") {
                t.Errorf("got err = %v; want out of range error", err)
        }
}


It then panics with:

--- PASS: TestUnmarshalJSONLiteralError (0.00 seconds)
panic: parsing time "0000-00-00T00:00:00Z": month out of range [recovered]
    panic: parsing time "0000-00-00T00:00:00Z": month out of range

goroutine 22 [running]:
testing.func·004(0x10384fa8, 0x10384100)
    /Users/bradfitz/go/src/pkg/testing/testing.go:309 +0xb3
encoding/json.(*decodeState).error(0xc2000ff800, 0xc200383c30, 0xc200181460, 0x16)
    /Users/bradfitz/go/src/pkg/encoding/json/decode.go:203 +0x47
encoding/json.(*decodeState).literalStore(0xc2000ff800, 0xc2000fe660, 0x16, 0x16,
0x18ea08, ...)
    /Users/bradfitz/go/src/pkg/encoding/json/decode.go:621 +0x286
encoding/json.Unmarshal(0xc2000fe660, 0x16, 0x16, 0x177d98, 0xc2000fe640, ...)
    /Users/bradfitz/go/src/pkg/encoding/json/decode.go:82 +0x266
encoding/json.TestUnmarshalJSONLiteralError(0xc2002d0750, 0x1dd35e08)
    /Users/bradfitz/go/src/pkg/encoding/json/decode_test.go:1136 +0xbd
testing.tRunner(0xc2002d0750, 0x2b91a8, 0x0, 0x0)
    /Users/bradfitz/go/src/pkg/testing/testing.go:314 +0x6c
created by testing.RunTests
    /Users/bradfitz/go/src/pkg/testing/testing.go:429 +0x875

goroutine 1 [chan receive]:
testing.RunTests(0x2000, 0x2b8fe0, 0x29, 0x29, 0x1, ...)
    /Users/bradfitz/go/src/pkg/testing/testing.go:430 +0x898
testing.Main(0x2000, 0x2b8fe0, 0x29, 0x29, 0x2b6350, ...)
    /Users/bradfitz/go/src/pkg/testing/testing.go:326 +0x8a
main.main()
    encoding/json/_test/_testmain.go:149 +0x9a
exit status 2
FAIL    encoding/json   0.101s