cmd/vet: don't exit with failure on type checking error

The vet tool only reports a type checking error when invoked with -v.
Don't let that by itself cause vet to exit with an error exit status.

Updates #21188

Change-Id: I172c13d46c35d49e229e96e833683d8c82a77de7
Reviewed-on: https://go-review.googlesource.com/52851
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/main.go

@@ -349,8 +349,9 @@ func doPackage(directory string, names []string, basePkg *Package) *Package {
 	pkg.files = files
 	// Type check the package.
 	err := pkg.check(fs, astFiles)
-	if err != nil && *verbose {
+	if err != nil {
-		warnf("%s", err)
+		// Note that we only report this error when *verbose.
+		Println(err)
 	}
 
 	// Check.

src/cmd/vet/testdata/cgo/cgo3.go

@@ -0,0 +1,13 @@
+// Copyright 2017 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.
+
+// Used by TestVetVerbose to test that vet -v doesn't fail because it
+// can't find "C".
+
+package testdata
+
+import "C"
+
+func F() {
+}

src/cmd/vet/vet_test.go

@@ -205,3 +205,15 @@ func TestTags(t *testing.T) {
 		})
 	}
 }
+
+// Issue #21188.
+func TestVetVerbose(t *testing.T) {
+	t.Parallel()
+	Build(t)
+	cmd := exec.Command("./"+binary, "-v", "-all", "testdata/cgo/cgo3.go")
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Logf("%s", out)
+		t.Error(err)
+	}
+}