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encoding/json: Avoid "invalid use of ,string struct tag, trying to unmarshal %q into %v" when target type matches #40266

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andig opened this issue Jul 17, 2020 · 2 comments

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@andig
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@andig andig commented Jul 17, 2020

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

$ go version
go version go1.14.4 darwin/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
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/andig/Library/Caches/go-build"
GOENV="/Users/andig/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/andig/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.14.4/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.14.4/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/andig/htdocs/evcc/go.mod"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/73/89ycv7qn51j4kbm04jsz9b840000gn/T/go-build573766352=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Unmarshal

{ "ser":"<serial>" }

into

type api struct { Serial string `json:ser,string` }

What did you expect to see?

No error

What did you see instead?

invalid use of ,string struct tag, trying to unmarshal "ser" into api.Serial"

While the error can easily be worked around by omitting the string tag, it shouldn't error as the types are compatible. That is- unless this error message is intended to ensure that the json user is 100% aware of the json schema he consumes.

@seankhliao
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@seankhliao seankhliao commented Jul 17, 2020

The string option has the specific meaning of representing a double encoded value, not just mangle it into a string

@mvdan
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@mvdan mvdan commented Jul 17, 2020

What @seankhliao said. A valid input would be like:

{ "ser": "\"doubly encoded string\"" }
@mvdan mvdan closed this Jul 17, 2020
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