internal/filetypes: File.Default is indeed not unused

We just didn't have test cases for it in this package.
Removing the unification broke plenty of cmd/cue tests.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I1905ca3e32e1bc1a82c85d46ef6560775c9be36d
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1193533
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>

internal/filetypes/filetypes.go

@@ -271,7 +271,6 @@ func toFile(modeVal, fileVal cue.Value, filename string) (*build.File, error) {
 	if concrete, hasDefault := hasEncoding(fileVal); !concrete {
 		if filename == "-" {
 			if !hasDefault {
-				// TODO(mvdan): unused?
 				fileVal = fileVal.Unify(modeVal.LookupPath(cue.MakePath(cue.Str("Default"))))
 			}
 		} else if ext := fileExt(filename); ext != "" {

internal/filetypes/filetypes_test.go

@@ -333,6 +333,20 @@ func TestParseFile(t *testing.T) {
 	}, {
 		in:  "file.bar",
 		out: `unknown file extension .bar`,
+	}, {
+		in:   "-",
+		mode: Input,
+		out: &build.File{
+			Filename: "-",
+			Encoding: "cue",
+		},
+	}, {
+		in:   "-",
+		mode: Export,
+		out: &build.File{
+			Filename: "-",
+			Encoding: "json",
+		},
 	}}
 	for _, tc := range testCases {
 		t.Run(tc.in, func(t *testing.T) {