1 files changed, 43 insertions, 0 deletions
diff --git a/src/io/fs/readfile_test.go b/src/io/fs/readfile_test.go
new file mode 100644
index 0000000000..0afa334ace
--- /dev/null
+++ b/src/io/fs/readfile_test.go
@@ -0,0 +1,43 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs_test
+
+import (
+	. "io/fs"
+	"testing"
+	"testing/fstest"
+	"time"
+)
+
+var testFsys = fstest.MapFS{
+	"hello.txt": {
+		Data:    []byte("hello, world"),
+		Mode:    0456,
+		ModTime: time.Now(),
+		Sys:     &sysValue,
+	},
+}
+
+var sysValue int
+
+type readFileOnly struct{ ReadFileFS }
+
+func (readFileOnly) Open(name string) (File, error) { return nil, ErrNotExist }
+
+type openOnly struct{ FS }
+
+func TestReadFile(t *testing.T) {
+	// Test that ReadFile uses the method when present.
+	data, err := ReadFile(readFileOnly{testFsys}, "hello.txt")
+	if string(data) != "hello, world" || err != nil {
+		t.Fatalf(`ReadFile(readFileOnly, "hello.txt") = %q, %v, want %q, nil`, data, err, "hello, world")
+	}
+
+	// Test that ReadFile uses Open when the method is not present.
+	data, err = ReadFile(openOnly{testFsys}, "hello.txt")
+	if string(data) != "hello, world" || err != nil {
+		t.Fatalf(`ReadFile(openOnly, "hello.txt") = %q, %v, want %q, nil`, data, err, "hello, world")
+	}
+}