cmd/go: make 'go mod verify' report an error outside a module

Also, test that 'go mod download' without arguments reports an error.

Fixes #32027

Change-Id: I873fc59fba4c78ee2b4f49f0d846ee2ac0eee4db
Reviewed-on: https://go-review.googlesource.com/c/go/+/202697
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/modcmd/verify.go

@@ -43,7 +43,7 @@ func runVerify(cmd *base.Command, args []string) {
 		base.Fatalf("go mod verify: verify takes no arguments")
 	}
 	// Checks go mod expected behavior
-	if !modload.Enabled() {
+	if !modload.Enabled() || !modload.HasModRoot() {
 		if cfg.Getenv("GO111MODULE") == "off" {
 			base.Fatalf("go: modules disabled by GO111MODULE=off; see 'go help modules'")
 		} else {

src/cmd/go/testdata/script/mod_outside.txt

@@ -82,6 +82,10 @@ stderr 'cannot find main module'
 stderr 'cannot find main module'
 
 
+# 'go mod download' without arguments should report an error.
+! go mod download
+stderr 'no modules specified'
+
 # 'go mod download' should download exactly the requested module without dependencies.
 rm -r $GOPATH/pkg/mod/cache/download/example.com
 go mod download example.com/printversion@v1.0.0
@@ -92,15 +96,16 @@ exists $GOPATH/pkg/mod/cache/download/example.com/printversion/@v/v1.0.0.zip
 ! go mod download all
 stderr 'go: cannot match "all": working directory is not part of a module'
 
+
 # 'go mod vendor' should fail: it starts by clearing the existing vendor
 # directory, and we don't know where that is.
 ! go mod vendor
 stderr 'cannot find main module'
 
-# 'go mod verify' should succeed: we have no modules to verify.
-go mod verify
-stdout 'all modules verified'
-! stderr .
+
+# 'go mod verify' should fail: we have no modules to verify.
+! go mod verify
+stderr 'cannot find main module'
 
 
 # 'go get' without arguments implicitly operates on the main module, and thus