cmd/go: fails to run test in generics-using package that has a dep not in go.mod

[bradfitz]

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

Both:

• go version go1.20.4 darwin/arm64
• go version devel go1.21-5c51e9f45b Thu May 4 18:03:04 2023 +0000 darwin/arm64

Does this issue reproduce with the latest release?

Yes

What did you do?

I tried to run tests of a package in another module, where my current module didn't have those test deps in go.mod.

You get weird -lang mismatches (is it synthesizing some fake module behind the scenes and not filling something out?) instead of either working or failing nicely.

Repro at https://github.com/bradfitz/go-test-bug-repro

$ go version
go version go1.20.4 darwin/arm64

$ go test tailscale.com/net/dns/resolver
# github.com/miekg/dns
embedding interface element ~[]string requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/msg.go:549:12: type parameter requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/msg.go:549:14: embedding interface element ~[]byte | ~string requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/msg.go:553:16: type parameter requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/types.go:1605:17: type parameter requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/types.go:1605:19: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/types.go:1605:26: embedding interface element ~[]E requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/clientconfig.go:71:25: implicit function instantiation requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/defaults.go:211:12: implicit function instantiation requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/edns.go:523:63: implicit function instantiation requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/edns.go:523:63: too many errors
FAIL	tailscale.com/net/dns/resolver [build failed]
FAIL

$ go get github.com/miekg/[email protected]    # <---- the workaround

$ go test tailscale.com/net/dns/resolver
ok  	tailscale.com/net/dns/resolver	0.160s

$ go mod tidy

$ go test tailscale.com/net/dns/resolver
# github.com/miekg/dns
embedding interface element ~[]string requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/msg.go:549:12: type parameter requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/msg.go:549:14: embedding interface element ~[]byte | ~string requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/msg.go:553:16: type parameter requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/types.go:1605:17: type parameter requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/types.go:1605:19: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/types.go:1605:26: embedding interface element ~[]E requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/clientconfig.go:71:25: implicit function instantiation requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/defaults.go:211:12: implicit function instantiation requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/edns.go:523:63: implicit function instantiation requires go1.18 or later (-lang was set to go1.16; check go.mod)
../go/pkg/mod/github.com/miekg/[email protected]/edns.go:523:63: too many errors
FAIL	tailscale.com/net/dns/resolver [build failed]
FAIL

/cc @bcmills @matloob @ianlancetaylor

[bcmills]

This is an unfortunate failure mode of #56222.

From a build at CL 489075 I see:

~/tmp/go-test-bug-repro$ go mod why github.com/miekg/dns
# github.com/miekg/dns
go-test-bug-repro
tailscale.com/tsnet
tailscale.com/ipn/ipnlocal
tailscale.com/net/dns/resolver
tailscale.com/net/dns/resolver.test
github.com/miekg/dns

~/tmp/go-test-bug-repro$ go test tailscale.com/net/dns/resolver
# tailscale.com/net/dns/resolver
../../pkg/mod/[email protected]/net/dns/resolver/tsdns_server_test.go:13:2: github.com/miekg/[email protected]: missing go.sum entry for go.mod file; to add it:
        go mod download github.com/miekg/dns
FAIL    tailscale.com/net/dns/resolver [setup failed]
FAIL

~/tmp/go-test-bug-repro$ go mod tidy -go=1.21

~/tmp/go-test-bug-repro$ go test tailscale.com/net/dns/resolver
ok      tailscale.com/net/dns/resolver  0.030s

[bcmills]

Duplicate of #56222