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

$ go version
go version devel +4b3f04c

Does this issue reproduce with the latest release?

Yes.

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

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="~/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="~"
GOPROXY=""
GORACE=""
GOROOT="~/go"
GOTMPDIR=""
GOTOOLDIR="~/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
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-build224479533=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Read the documentation for build/test caching from go help test. Then

$ go test -tags noasm gonum.org/v1/gonum/mat
ok  	gonum.org/v1/gonum/mat	44.695s
$ go test -tags noasm gonum.org/v1/gonum/mat
ok  	gonum.org/v1/gonum/mat	(cached)

From this it's clear that -tags is not counted in test flags (and this is reasonably given the wording).

What did you expect to see?

No caching.

What did you see instead?

Happily, caching.

While the wording is consistent with the apparent intended meaning, it could be made clearer that build flags do not impact on caching for tests (if this is true - I could find no mention of caching in go help build or go help install or mention of which flags prevent caching (other than -a in go help cache)).