url.Parse() wrongly assumes scheme for schema-less hosts with port

[iwyg]

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

$ go version go1.16.6 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="~/Library/Caches/go-build"
GOENV="~/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="~/code/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="~/code/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.16.6/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.16.6/libexec/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.16.6"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="~/code/go/src/gitlab.tmt.de/streaMonkey/xstream/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"

What did you do?

https://play.golang.org/p/EEzkqs1o0OS

What did you expect to see?

I expect to either an error, or an url like

&url.URL{Scheme:"", Opaque:"", User:(*url.Userinfo)(nil), Host:"localhost:8080", Path:"", RawPath:"", ForceQuery:false, RawQuery:"", Fragment:"", RawFragment:""}

What did you see instead?

&url.URL{Scheme:"localhost", Opaque:"8080", User:(*url.Userinfo)(nil), Host:"", Path:"", RawPath:"", ForceQuery:false, RawQuery:"", Fragment:"", RawFragment:""}

[seankhliao]

Note it does say

Trying to parse a hostname and path without a scheme is invalid but may not necessarily return an error, due to parsing ambiguities.

[seankhliao]

Duplicate of #21415