cmd/vet: check for nil result set in isStringer

Fixes golang/go#7149.

LGTM=gri
R=golang-codereviews, gri
CC=golang-codereviews
https://golang.org/cl/68260044

cmd/vet/doc.go

@@ -22,7 +22,7 @@ vets the files named, all of which must be in the same package.
 By directory:
 	go tool vet source/directory
 recursively descends the directory, vetting each file in isolation.
-Package-level type-checking disabled, so the vetting is weaker.
+Package-level type-checking is disabled, so the vetting is weaker.
 
 Vet's exit code is 2 for erroneous invocation of the tool, 1 if a
 problem was reported, and 0 otherwise. Note that the tool does not

cmd/vet/main.go

@@ -76,7 +76,8 @@ func Usage() {
 	fmt.Fprintf(os.Stderr, "\tvet [flags] directory...\n")
 	fmt.Fprintf(os.Stderr, "\tvet [flags] files... # Must be a single package\n")
 	fmt.Fprintf(os.Stderr, "For more information run\n")
-	fmt.Fprintf(os.Stderr, "\tgodoc code.google.com/p/go.tools/cmd/vet\n\nFlags:\n")
+	fmt.Fprintf(os.Stderr, "\tgodoc code.google.com/p/go.tools/cmd/vet\n\n")
+	fmt.Fprintf(os.Stderr, "Flags:\n")
 	flag.PrintDefaults()
 	os.Exit(2)
 }
@@ -457,7 +458,7 @@ func (f *File) prepStringerReceiver(d *ast.FuncDecl) {
 // isStringer returns true if the provided declaration is a "String() string"
 // method; an implementation of fmt.Stringer.
 func (f *File) isStringer(d *ast.FuncDecl) bool {
-	return d.Recv != nil && d.Name.Name == "String" &&
+	return d.Recv != nil && d.Name.Name == "String" && d.Type.Results != nil &&
 		len(d.Type.Params.List) == 0 && len(d.Type.Results.List) == 1 &&
 		f.pkg.types[d.Type.Results.List[0].Type].Type == types.Typ[types.String]
 }

cmd/vet/testdata/print.go

@@ -323,3 +323,8 @@ type RecursiveStruct2 struct {
 }
 
 var recursiveStruct1V = &RecursiveStruct1{}
+
+// Fix for issue 7149: Missing return type on String method caused fault.
+func (int) String() {
+	return ""
+}