encoding/json: TextMarshaler type map key is not supported

[AllenX2018]

What version of Go are you using (go version)?

$ go version
go version go1.14.1 windows/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
set GO111MODULE=on
set GOARCH=amd64
set GOBIN=
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows

What did you do?

https://play.golang.org/p/ArLLY9n2uo3

What did you expect to see?

Output:{"MANUAL__aaa":"bbb"}

What did you see instead?

Output:{"aaa":"bbb"}

[mvdan]

This is on purpose and documented; see #28827.

[mvdan]

Here's the order, as per the docs:

- keys of any string type are used directly
- encoding.TextMarshalers are marshaled
- integer keys are converted to strings

To force json to use your marshaler, don't let your type be a string type: https://play.golang.org/p/xMmVspIdMMS

You could argue the first two options should be reversed in the order, but I don't think we can change the documented behavior at this point without breaking a good portion of users.