cmd/link: add a test that reflect.Value.Call does not bring methods live

reflect.Value.Call, if reachable, used to bring all exported
methods live. CL 228792 fixes this, removing the check of
reflect.Value.Call. This CL adds a test.

Updates #38505.

Change-Id: Ib4cab3c3c86c9c9702d041266e59b159d0ff0a97
Reviewed-on: https://go-review.googlesource.com/c/go/+/228878
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/link/internal/ld/deadcode_test.go

@@ -0,0 +1,61 @@
+// 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 ld
+
+import (
+	"bytes"
+	"internal/testenv"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"testing"
+)
+
+// This example uses reflect.Value.Call, but not
+// reflect.{Value,Type}.Method. This should not
+// need to bring all methods live.
+const deadcodeTestSrc = `
+package main
+import "reflect"
+
+func f() { println("call") }
+
+type T int
+func (T) M() {}
+
+func main() {
+	v := reflect.ValueOf(f)
+	v.Call(nil)
+	i := interface{}(T(1))
+	println(i)
+}
+`
+
+func TestDeadcode(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	tmpdir, err := ioutil.TempDir("", "TestDeadcode")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpdir)
+
+	src := filepath.Join(tmpdir, "main.go")
+	err = ioutil.WriteFile(src, []byte(deadcodeTestSrc), 0666)
+	if err != nil {
+		t.Fatal(err)
+	}
+	exe := filepath.Join(tmpdir, "main.exe")
+
+	cmd := exec.Command(testenv.GoToolPath(t), "build", "-ldflags=-dumpdep", "-o", exe, src)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("%v: %v:\n%s", cmd.Args, err, out)
+	}
+	if bytes.Contains(out, []byte("main.T.M")) {
+		t.Errorf("main.T.M should not be reachable. Output:\n%s", out)
+	}
+}