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

$ go version
go version go1.14 linux/amd64

Does this issue reproduce with the latest release?

I don't know

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

$ go env
fred@fred-linux:~/src/server/build$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/fred/.cache/go-build"
GOENV="/home/fred/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/fred/go:/home/fred/src/server/Gofred/"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/linuxbrew/.linuxbrew/Cellar/go/1.14/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/linuxbrew/.linuxbrew/Cellar/go/1.14/libexec/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc-5"
CXX="g++-5"
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-build142698688=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I probably had gcc-5 as the default compiler on my ubuntu system. Then I did change the default compiler (with update-alternative) so that gcc now invokes gcc-8 (same for g++).

I have some c++ code I try to bridge with cgo that relies on c++ 17. I try to build it this way

 go build -x  -o /home/fred/src/server/build/libcppgow.so -buildmode=c-shared main.go

The output shows me it is using g++-5 (and the build fails, of course). go env also shows:

 CC="gcc-5"
 CXX="g++-5"

Somehow, go does not want to use the default compiler. He might have cache somewhere the default compiler from an earlier invocation. Or it might use the compiler at the time I installed go (using brew).

What did you expect to see?

I expect the CC/CXX are inferred to be the default compiler of the platform.
If I'm wrong, then you should clearly document how CC/CXX are inferred.