cmd/go: go mod gets dependency, which can not built, because needs newer go version

[Ragnar-BY]

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

$go version go1.15.2 linux/amd64

Does this issue reproduce with the latest release?

I don`t know

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/user/.cache/go-build"
GOENV="/home/user/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/user/go/pkg/mod"
GONOPROXY="git.dar.kz"
GONOSUMDB="git.dar.kz"
GOOS="linux"
GOPATH="/home/user/go"
GOPRIVATE="git.dar.kz"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
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-build726176060=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I have project which is built with go1.15 and use go modules
My project has labstack/echo as dependency
And after go mod tidy
I have

go.mod

module myproject
go 1.15
require (

...

github.com/labstack/echo/v4 v4.3.0

...

)

But this version of labstack/echo is built with go1.16, and I get next error

myproject imports
        github.com/labstack/echo/v4/middleware tested by
        github.com/labstack/echo/v4/middleware.test imports
        io/fs: package io/fs is not in GOROOT (/usr/local/go/src/io/fs)
myproject imports
        github.com/labstack/echo/v4/middleware tested by
        github.com/labstack/echo/v4/middleware.test imports
        testing/fstest: package testing/fstest is not in GOROOT (/usr/local/go/src/testing/fstest)

What did you expect to see?

I expect, that go mod will get version of modules, that can be built with same go version as mine

What did you see instead?

I get module, that built with go1.16, but I need module for go1.15

I can fix it but changing go.mod file and downgrading version

[cherrymui]

Does github.com/labstack/echo/v4 v4.3.0 actually depends on Go 1.16? If so, I'm not sure there is anything we can do here.

cc @bcmills @jayconrod @matloob

[bcmills]

@Ragnar-BY, upgrade to Go 1.15.10.

[bcmills]

Duplicate of #44792