cmd/vet: unexported Stringer and error fields cannot be formatted

According to CL 31817, fmt cannot invoke String or Error methods
on unexported struct fields.

Fixes #17798.

Change-Id: I0d516577298bc36daa9a94313c3874d64dc079e6
Reviewed-on: https://go-review.googlesource.com/44831Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/testdata/print.go

@@ -403,7 +403,7 @@ var notPercentDV notPercentDStruct
 type percentSStruct struct {
 	a string
 	b []byte
-	c stringerarray
+	C stringerarray
 }
 
 var percentSV percentSStruct
@@ -472,3 +472,54 @@ func UnknownStructFprintln() {
 	s := unknownStruct{}
 	s.Fprintln(os.Stdout, "hello, world!") // OK
 }
+
+// Issue 17798: unexported stringer cannot be formatted.
+type unexportedStringer struct {
+	t stringer
+}
+type unexportedStringerOtherFields struct {
+	s string
+	t stringer
+	S string
+}
+
+// Issue 17798: unexported error cannot be formatted.
+type unexportedError struct {
+	e error
+}
+type unexportedErrorOtherFields struct {
+	s string
+	e error
+	S string
+}
+
+type errorer struct{}
+
+func (e errorer) Error() string { return "errorer" }
+
+func UnexportedStringerOrError() {
+	us := unexportedStringer{}
+	fmt.Printf("%s", us)  // ERROR "arg us for printf verb %s of wrong type"
+	fmt.Printf("%s", &us) // ERROR "arg &us for printf verb %s of wrong type"
+
+	usf := unexportedStringerOtherFields{
+		s: "foo",
+		S: "bar",
+	}
+	fmt.Printf("%s", usf)  // ERROR "arg usf for printf verb %s of wrong type"
+	fmt.Printf("%s", &usf) // ERROR "arg &usf for printf verb %s of wrong type"
+
+	ue := unexportedError{
+		e: &errorer{},
+	}
+	fmt.Printf("%s", ue)  // ERROR "arg ue for printf verb %s of wrong type"
+	fmt.Printf("%s", &ue) // ERROR "arg &ue for printf verb %s of wrong type"
+
+	uef := unexportedErrorOtherFields{
+		s: "foo",
+		e: &errorer{},
+		S: "bar",
+	}
+	fmt.Printf("%s", uef)  // ERROR "arg uef for printf verb %s of wrong type"
+	fmt.Printf("%s", &uef) // ERROR "arg &uef for printf verb %s of wrong type"
+}

src/cmd/vet/types.go

@@ -134,10 +134,8 @@ func (f *File) matchArgTypeInternal(t printfArgType, typ types.Type, arg ast.Exp
 		return true
 	}
 	// If we can use a string, might arg (dynamically) implement the Stringer or Error interface?
-	if t&argString != 0 {
-		if types.AssertableTo(errorType, typ) || stringerType != nil && types.AssertableTo(stringerType, typ) {
-			return true
-		}
+	if t&argString != 0 && isConvertibleToString(typ) {
+		return true
 	}
 
 	typ = typ.Underlying()
@@ -261,6 +259,10 @@ func (f *File) matchArgTypeInternal(t printfArgType, typ types.Type, arg ast.Exp
 	return false
 }
 
+func isConvertibleToString(typ types.Type) bool {
+	return types.AssertableTo(errorType, typ) || stringerType != nil && types.AssertableTo(stringerType, typ)
+}
+
 // hasBasicType reports whether x's type is a types.Basic with the given kind.
 func (f *File) hasBasicType(x ast.Expr, kind types.BasicKind) bool {
 	t := f.pkg.types[x].Type
@@ -275,7 +277,12 @@ func (f *File) hasBasicType(x ast.Expr, kind types.BasicKind) bool {
 // type. For instance, with "%d" all the elements must be printable with the "%d" format.
 func (f *File) matchStructArgType(t printfArgType, typ *types.Struct, arg ast.Expr, inProgress map[types.Type]bool) bool {
 	for i := 0; i < typ.NumFields(); i++ {
-		if !f.matchArgTypeInternal(t, typ.Field(i).Type(), arg, inProgress) {
+		typf := typ.Field(i)
+		if !f.matchArgTypeInternal(t, typf.Type(), arg, inProgress) {
+			return false
+		}
+		if t&argString != 0 && !typf.Exported() && isConvertibleToString(typf.Type()) {
+			// Issue #17798: unexported Stringer or error cannot be properly fomatted.
 			return false
 		}
 	}