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

go version go1.15.5 linux/amd64

Does this issue reproduce with the latest release?

yes

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

$ go env
circleci@0b9c61f00e98:/go$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/circleci/.cache/go-build"
GOENV="/home/circleci/.config/go/env"
GOEXE=""
GOFLAGS="-mod=vendor"
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
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-build517168634=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I'm using packages.Load (golang.org/x/tools/go/packages) to collect source files of go packages.
My program is calling packagesLoad() with Config.Env set to []string{"GO111MODULE=on", "GOFLAGS=-mod=readonly"}.
I ran the program in an environment where the environment variable GOPATH was set to /go.

What did you expect to see?

I expected that packages.Load() runs go list without the GOPATH variable of my current environment, instead that either GOPATH is unset or the default value is used.

I expected this because the documentation of Config.Env states:

// Env is the environment to use when invoking the build system's query tool.
// If Env is nil, the current environment is used.

https://github.com/golang/tools/blob/eeaa07dd769687291ac8e71ec39a7db382b22b65/go/packages/packages.go#L130

What did you see instead?

The setting of my current environment were used for the go list execution.
go list is run via Invocation.run() which uses the current environment and appends additional environment variables to the slice:

	cmd.Env = append(os.Environ(), i.Env...)

https://github.com/golang/tools/blob/eeaa07dd769687291ac8e71ec39a7db382b22b65/internal/gocommand/invoke.go#L210

That Invocation.run() always uses the current environment was introduced in commit:
golang/tools@f8bfb4e#diff-55f055b305279edb4e62ce29481a985df8d488314120de25803e0336efdea6e1

I would prefer if instead of correcting the documentation, packages.Load() would work as it is currently documented.
It makes it easy to run the function in a controlled environment.