proxy.golang.org: non-ascii paths not supported

[carnott-snap]

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?

Yes.

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

go env Output

$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/user/.cache/go-build"
GOENV="/home/user/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOOS="linux"
GOPATH="/home/user/go"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/user/.local/share/umake/go/go-lang"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/user/.local/share/umake/go/go-lang/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
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-build606606876=/tmp/go-build -gno-record-gcc-switches"

What did you do?

package main

import _ "example.com/aаa"

go mod tidy

What did you expect to see?

go: finding module for package example.com/aаa
main imports
        example.com/aаa: cannot find module providing package example.com/aаa: unrecognized import path "example.com/aаa": reading https://example.com/a%D0%B0a?go-get=1: 404 Not Found

What did you see instead?

go: finding module for package example.com/aаa
main imports
        example.com/aаa: malformed module path "example.com/aаa": invalid char 'а'

[bcmills]

I would argue that we should never accept the particular path example.com/aаa: it is indistinguishable from a homograph attack.

For other unicode paths, see #29101.

[bcmills]

Duplicate of #29101