cmd/vet: %s is valid for an array of stringer

vet was quiet for []stringer, but not for [N]stringer. The source of the
problem was how the recursive call used .Elem().Underlying() for arrays,
but .Elem() for slices. In the first case, the named type is dropped,
thus losing all information of attached methods.

Be consistent across slices and arrays, by dropping the Underlying call
that is causing trouble. Add regression tests too, including cases where
the element type does not implement fmt.Stringer.

Fixes #23552.

Change-Id: I0fde07d101f112d5768be0a79207ef0b3dc45f2e
Reviewed-on: https://go-review.googlesource.com/90455
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/testdata/print.go

@@ -130,8 +130,8 @@ func PrintfTests() {
 	fmt.Printf("%U", x)                         // ERROR "Printf format %U has arg x of wrong type float64"
 	fmt.Printf("%x", nil)                       // ERROR "Printf format %x has arg nil of wrong type untyped nil"
 	fmt.Printf("%X", 2.3)                       // ERROR "Printf format %X has arg 2.3 of wrong type float64"
-	fmt.Printf("%s", stringerv)                 // ERROR "Printf format %s has arg stringerv of wrong type testdata.stringer"
-	fmt.Printf("%t", stringerv)                 // ERROR "Printf format %t has arg stringerv of wrong type testdata.stringer"
+	fmt.Printf("%s", stringerv)                 // ERROR "Printf format %s has arg stringerv of wrong type testdata.ptrStringer"
+	fmt.Printf("%t", stringerv)                 // ERROR "Printf format %t has arg stringerv of wrong type testdata.ptrStringer"
 	fmt.Printf("%s", embeddedStringerv)         // ERROR "Printf format %s has arg embeddedStringerv of wrong type testdata.embeddedStringer"
 	fmt.Printf("%t", embeddedStringerv)         // ERROR "Printf format %t has arg embeddedStringerv of wrong type testdata.embeddedStringer"
 	fmt.Printf("%q", notstringerv)              // ERROR "Printf format %q has arg notstringerv of wrong type testdata.notstringer"
@@ -168,7 +168,7 @@ func PrintfTests() {
 	Printf(format, "hi", "there")
 	Printf(format, "hi")              // ERROR "Printf format %s reads arg #2, but call has only 1 arg$"
 	Printf("%s %d %.3v %q", "str", 4) // ERROR "Printf format %.3v reads arg #3, but call has only 2 args"
-	f := new(stringer)
+	f := new(ptrStringer)
 	f.Warn(0, "%s", "hello", 3)           // ERROR "Warn call has possible formatting directive %s"
 	f.Warnf(0, "%s", "hello", 3)          // ERROR "Warnf call needs 1 arg but has 2 args"
 	f.Warnf(0, "%r", "hello")             // ERROR "Warnf format %r has unknown verb r"
@@ -352,25 +352,29 @@ func multi() []interface{} {
 	panic("don't call - testing only")
 }
 
-type stringer float64
+type stringer int
 
-var stringerv stringer
+func (stringer) String() string { return "string" }
 
-func (*stringer) String() string {
+type ptrStringer float64
+
+var stringerv ptrStringer
+
+func (*ptrStringer) String() string {
 	return "string"
 }
 
-func (*stringer) Warn(int, ...interface{}) string {
+func (*ptrStringer) Warn(int, ...interface{}) string {
 	return "warn"
 }
 
-func (*stringer) Warnf(int, string, ...interface{}) string {
+func (*ptrStringer) Warnf(int, string, ...interface{}) string {
 	return "warnf"
 }
 
 type embeddedStringer struct {
 	foo string
-	stringer
+	ptrStringer
 	bar int
 }
 
@@ -479,13 +483,13 @@ type RecursiveStruct2 struct {
 
 var recursiveStruct1V = &RecursiveStruct1{}
 
-// Issue 17798: unexported stringer cannot be formatted.
+// Issue 17798: unexported ptrStringer cannot be formatted.
 type unexportedStringer struct {
-	t stringer
+	t ptrStringer
 }
 type unexportedStringerOtherFields struct {
 	s string
-	t stringer
+	t ptrStringer
 	S string
 }
 
@@ -533,6 +537,13 @@ func UnexportedStringerOrError() {
 	fmt.Println("foo\n")        // ERROR "Println arg list ends with redundant newline"
 	fmt.Println("foo\\n")       // not an error
 	fmt.Println(`foo\n`)        // not an error
+
+	intSlice := []int{3, 4}
+	fmt.Printf("%s", intSlice) // ERROR "Printf format %s has arg intSlice of wrong type \[\]int"
+	nonStringerArray := [1]unexportedStringer{{}}
+	fmt.Printf("%s", nonStringerArray)  // ERROR "Printf format %s has arg nonStringerArray of wrong type \[1\]testdata.unexportedStringer"
+	fmt.Printf("%s", []stringer{3, 4})  // not an error
+	fmt.Printf("%s", [2]stringer{3, 4}) // not an error
 }
 
 // TODO: Disable complaint about '0' for Go 1.10. To be fixed properly in 1.11.

src/cmd/vet/types.go

@@ -172,7 +172,7 @@ func (f *File) matchArgTypeInternal(t printfArgType, typ types.Type, arg ast.Exp
 			return true // %s matches []byte
 		}
 		// Recur: []int matches %d.
-		return t&argPointer != 0 || f.matchArgTypeInternal(t, typ.Elem().Underlying(), arg, inProgress)
+		return t&argPointer != 0 || f.matchArgTypeInternal(t, typ.Elem(), arg, inProgress)
 
 	case *types.Slice:
 		// Same as array.