Please answer these questions before submitting your issue. Thanks!

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

go version go1.8 darwin/amd64

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/mdee/data/go_third_party_local:/Users/mdee/data/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/rp/m0d1xlms1ts618xx3x1vgjp80000gp/T/go-build670327831=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

What did you do?

Used a sql.DB whose driver does not implement ConnBeginTx (in my case, an outdated version of pq), and begun a transaction with
db.BeginTx(context.TODO, &sql.TxOptions{ReadOnly: true}).

What did you expect to see?

I expected to see an error, since the driver does not support controlling ReadOnly or Isolation

What did you see instead?

This successfully opens a transaction which is not readonly. Similarly, any isolation levels will be ignored.