show error when go modules is used with --with-dep

generators/newapp/generator.go

@@ -22,6 +22,8 @@ var Templates = packr.NewBox("../newapp/templates")
 // ErrNotInGoPath can be asserted against
 var ErrNotInGoPath = errors.New("currently not in a $GOPATH")
 
+var errGoModulesWithDep = errors.New("--with-dep cannot be used with Go Modules enabled")
+
 // Generator is the representation of a new Buffalo application
 type Generator struct {
 	meta.App
@@ -118,9 +120,14 @@ func (g Generator) Validate() error {
 }
 
 func (g Generator) validateInGoPath() error {
+	if g.App.WithModules && g.App.WithDep {
+		return errGoModulesWithDep
+	}
+
 	if g.App.WithModules {
 		return nil
 	}
+
 	gpMultiple := envy.GoPaths()
 
 	larp := strings.ToLower(meta.ResolveSymlinks(filepath.Dir(g.Root)))

generators/newapp/generator_test.go

@@ -3,6 +3,7 @@ package newapp
 import (
 	"testing"
 
+	"github.com/gobuffalo/buffalo/meta"
 	"github.com/gobuffalo/packr"
 	"github.com/pkg/errors"
 	"github.com/stretchr/testify/require"
@@ -31,3 +32,29 @@ func Test_Validate_TemplatesMissing(t *testing.T) {
 	r.Error(err)
 	r.Equal(ErrTemplatesNotFound, errors.Cause(err))
 }
+
+func Test_Validate_InGoPath(t *testing.T) {
+	r := require.New(t)
+
+	tests := []struct {
+		g   Generator
+		err error
+	}{
+		{
+			g: Generator{
+				App: meta.App{WithModules: true},
+			},
+			err: nil,
+		},
+		{
+			g: Generator{
+				App: meta.App{WithModules: true, WithDep: true},
+			},
+			err: errGoModulesWithDep,
+		},
+	}
+
+	for _, test := range tests {
+		r.Equal(test.err, test.err)
+	}
+}