encoding/json: bug in json.Unmarshal if keys in JSON differ only by case

[AyushG3112]

Please answer these questions before submitting your issue. Thanks!

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

Playground and go version go1.10.2 linux/amd64

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

Playground and:

GOARCH="amd64"
GOBIN=""
GOCACHE= **REDACTED**
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH= **REDACTED**
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build198743377=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Here is the input json

{"a" :"This is value for ASmall", "A": "This is value for ACaps"}

In Playground Link 1, my target struct only has a field corresponding to the a key.

In Playground Link 2, my target struct has fields corresponding to both a and A.

What did you expect to see?

In Link 1, I expected to see the value printed to be This is value for ASmall which corresponds to the value of key a

What did you see instead?

The value printed was This is value for ACaps which corresponds to the value of key A.
Link 2 worked fine.

[mvdan]

Reading the godoc, this appears to be incorrect behavior:

To unmarshal JSON into a struct, Unmarshal matches incoming object keys to the keys used by Marshal (either the struct field name or its tag), preferring an exact match but also accepting a case-insensitive match.

If the case insensitive match overwrites the case sensitive match, it seems like the preference mentioned above isn't working.

[AyushG3112]

@mvdan Interestingly, if I switch the order of keys in the JSON, so that the new JSON becomes:

{"A":"This is value for ACaps","a":"This is value for ASmall"}

It works fine. Playground link

[AlexRouSg]

@mvdan I believe this is #14750

[agnivade]

/cc @dsnet

[mvdan]

Indeed seems like a duplicate - thanks @AlexRouSg.

[nikitml]

@AyushG3112 not it's not - https://play.golang.org/p/uyYYa186mez - the latter is used

[AyushG3112]

@npenzin I already saw that, refer my first reply to this issue after the OP. But again, for a valid JSON with a valid target struct, the order of keys should not matter.