encoding/json: MarshalJSON not called in value contexts for integer types

[schani]

This behavior is counterintuitive and requires bookkeeping to figure out which types are in a value context under which circumstances.

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

go version go1.9.2 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/schani/go"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/n9/hkp85g8d1_ddvqjfd9b_2b1h0000gn/T/go-build054789224=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
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"

What did you do?

https://play.golang.org/p/HuQg9qdj4HE

What did you expect to see?

{"map":{"a":"string"},"direct":"string"}
{"map":{"a":"string"},"direct":"string"}

What did you see instead?

{"map":{"a":0},"direct":0}
{"map":{"a":0},"direct":"string"}

[dsnet]

The issue is that MarshalJSON is defined as a method on the pointer version of the integer. It should probably be the value receiver. Closing as duplicate of #22967

[bradfitz]

Also note that elements in maps aren't addressable so any typical automatic T to *T receiver switch sugar doesn't happen since the map element can't have its pointer addr taken.