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

Tested with go1.20 and go1.21

Does this issue reproduce with the latest release?

yes

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

Included, though I doubt it has an impact:

$ go env
GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='$HOME/Library/Caches/go-build'
GOENV='$HOME/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='$HOME/go/pkg/mod'
GONOPROXY='corp.host/repos/*'
GONOSUMDB='corp.host/repos/*'
GOOS='darwin'
GOPATH='$HOME/go'
GOPRIVATE='corp.host/repos/*'
GOPROXY='https://proxy.golang.org,direct'
GOROOT='$HOME/sdk/go1.21.0'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='$HOME/sdk/go1.21.0/pkg/tool/darwin_amd64'
GOVCS=''
GOVERSION='go1.21.0'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='$HOME/go/src/corp.host/repos/blah/blah/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/mx/0hyblhts2jq55slmx983zjl80000gn/T/go-build2461970070=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

The documentation for build constraints states that constraints

must appear near the top of the file, preceded only by blank lines and other line comments.

However, during experimentation it became apparent that //go:build directives are honored even after block
comments. This didn't seem to hold true for the older //+build form.

I reached out to gophers slack's #tools channel for their input, and they thought it'd be
worth filing an issue. dominikh created https://go.dev/play/p/5jqlmjNEjOH.go showing his
results, which match mine.

What did you expect to see?

That the compiler and documentation match. If the current docs are correct, the compiler would ignore build constraints if proceeded by things other than line comments, while if the current compiler is correct, that the docs describe what the compiler allows.

For example, the docs seem to imply that the constraint should be ignored:

/* block comment*/

//go:build ignore

package etaoin

func shrdlu() {}

What did you see instead?

//go:build is honored even if following block comments.

Note: the old style //+build is ignored after block comments, matching the documentation.