encoding/json: decoder does not prefer exact match

[andrealancichinetti]

Go version

go1.21.4 darwin/arm64 (but also go.1.22)

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/andrea/Library/Caches/go-build'
GOENV='/Users/andrea/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/andrea/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/andrea/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.21.4'
GCCGO='gccgo'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='/Users/andrea/Desktop/work/cedar/tms/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/l1/f52v6_6x7r3898mqkgxf_pbm0000gn/T/go-build4023306156=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

https://go.dev/play/p/vOphzSORsXL

What did you see happen?

error: {not an exact match with tag}

What did you expect to see?

I have a struct with a field whose tag is an exact match of the JSON key. However, the decoder picks the JSON key with a case-insensitive match.

[seankhliao]

the matching is a many to one mapping from json data to struct fields, in the order that the data is presented, so length matches and its value is assigned to the field, then it sees Length, which also matches the field so the data is overwritten (it doesn't record whether a field was seen before). You can observe the effect if you switch the order of the fields in json.

unfortunate, but I believe this is working as intended.

[andrealancichinetti]

Thanks Sean, I have now found out that there is a whole discussion about this here: #63397, which is pretty interesting