cmd/go: error if -modfile used with 'go install pkg@version'

'go install pkg@version' runs without a main module or a module root
directory. The -modfile flag cannot be used to set the module root
directory or to substitute a different go.mod file.

This error won't be reported if -modfile is set in
GOFLAGS. Unsupported flags there are generally ignored.

For #40276

Change-Id: I0b39b1fa9184c15c6e863b647d43c328710920f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/258297
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/cmd/go/internal/modload/init.go

@@ -163,8 +163,9 @@ func Init() {
 		// Running 'go mod init': go.mod will be created in current directory.
 		modRoot = base.Cwd
 	} else if RootMode == NoRoot {
-		// TODO(jayconrod): report an error if -mod -modfile is explicitly set on
-		// the command line. Ignore those flags if they come from GOFLAGS.
+		if cfg.ModFile != "" && !base.InGOFLAGS("-modfile") {
+			base.Fatalf("go: -modfile cannot be used with commands that ignore the current module")
+		}
 		modRoot = ""
 	} else {
 		modRoot = findModuleRoot(base.Cwd)

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

@@ -26,6 +26,17 @@ rm $GOPATH/bin/a
 cd ..
 
 
+# 'go install -modfile=x.mod pkg@version' reports an error, but only if
+# -modfile is specified explicitly on the command line.
+cd m
+env GOFLAGS=-modfile=go.mod
+go install example.com/cmd/a@latest  # same as above
+env GOFLAGS=
+! go install -modfile=go.mod example.com/cmd/a@latest
+stderr '^go: -modfile cannot be used with commands that ignore the current module$'
+cd ..
+
+
 # Every test case requires linking, so we only cover the most important cases
 # when -short is set.
 [short] stop