encoding/json: incorrect object key unmarshaling when using custom TextUnmarshaler as Key with string values [1.14 backport]

[gopherbot]

@mvdan requested issue #39555 to be considered for backport to the next 1.14 minor release.

Thank you for the report. This is indeed a different regression caused by the same original performance optimization, https://golang.org/cl/190659. The previous fix was good as far as I can tell, but it evidently wasn't a complete fix.

It's pretty obvious that the original optimization was a mistake, and applying more best-effort fixes seems too little and too late at this point. I'll send a CL to revert the entire thing, and add more tests. We can backport this into 1.14.5.

@gopherbot please consider this for backport to 1.14, it's a regression.

[gopherbot]

Change https://golang.org/cl/241081 mentions this issue: [release-branch.go1.14] encoding/json: revert "avoid work when unquoting strings, take 2"

[dmitshur]

Approved as this 1.14-only regression is a serious issue without a workaround.

[gopherbot]

Closed by merging be0254a to release-branch.go1.14.