encoding/json: UnmarshalText confuses json.Unmarshal

[kortschak]

What does 'go version' print?
go version go1.3 linux/amd64
go version devel +f7e7857afd88 Fri Aug 01 16:45:33 2014 -0700 linux/amd64

What steps reproduce the problem?
If possible, include a link to a program on play.golang.org.

1. Run the program at http://play.golang.org/p/izafDRNZFW
2. Run the program at http://play.golang.org/p/Un74tdY10R

What happened?
The first program prints a reasonably unhelpful error "error: JSON decoder out of
sync - data changing underfoot?" and the second program prints the expected result.

What should have happened instead?
Both programs should behave as the second, a more informative error should be printed,
or the json documentation should warn about use of encoding.TextUnmarshaler.

[ianlancetaylor]

Comment 1:

Labels changed: added repo-main, release-none.

[rsc]

Dup of #9650.