handle the nil interface better in reflect and print

R=rsc
DELTA=25  (19 added, 0 deleted, 6 changed)
OCL=20985
CL=20985

src/lib/fmt/print.go

@@ -307,10 +307,13 @@ func parsenum(s string, start, end int) (n int, got bool, newi int) {
 }
 
 func (p *P) printField(field reflect.Value) (was_string bool) {
-	if stringer, ok := field.Interface().(String); ok {
+	inter := field.Interface();
+	if inter != nil {
+		if stringer, ok := inter.(String); ok {
 			p.addstr(stringer.String());
 			return false;	// this value is not a string
 		}
+	}
 	s := "";
 	switch field.Kind() {
 	case reflect.BoolKind:
@@ -363,6 +366,14 @@ func (p *P) printField(field reflect.Value) (was_string bool) {
 		p.add('{');
 		p.doprint(field, true, false);
 		p.add('}');
+	case reflect.InterfaceKind:
+		inter := field.(reflect.InterfaceValue).Get();
+		if inter == nil {
+			s = "<nil>"
+		} else {
+			// should never happen since a non-nil interface always has a type
+			s = "<non-nil interface>";
+		}
 	default:
 		s = "?" + field.Type().String() + "?";
 	}
@@ -421,8 +432,9 @@ func (p *P) doprintf(format string, v reflect.StructValue) {
 		}
 		field := getField(v, fieldnum);
 		fieldnum++;
-		if c != 'T' {	// don't want thing to describe itself if we're asking for its type
-			if formatter, ok := field.Interface().(Format); ok {
+		inter := field.Interface();
+		if inter != nil && c != 'T' {	// don't want thing to describe itself if we're asking for its type
+			if formatter, ok := inter.(Format); ok {
 				formatter.Format(p, c);
 				continue;
 			}

src/lib/reflect/type.go

@@ -340,6 +340,8 @@ func (t *InterfaceTypeStruct) Len() int {
 	return len(t.field)
 }
 
+var NilInterface = NewInterfaceTypeStruct("nil", "", new([]Field, 0));
+
 // -- Func
 
 export type FuncType interface {
@@ -834,6 +836,10 @@ func (p *Parser) Type(name string) *StubType {
 }
 
 export func ParseTypeString(name, typestring string) Type {
+	if typestring == "" {
+		// If the typestring is empty, it represents (the type of) a nil interface value
+		return NilInterface
+	}
 	p := new(Parser);
 	p.str = typestring;
 	p.Next();

src/lib/reflect/value.go

@@ -59,6 +59,7 @@ type Creator *(typ Type, addr Addr) Value
 export type MissingValue interface {
 	Kind()	int;
 	Type()	Type;
+	Addr()	Addr;
 }
 
 type MissingValueStruct struct {
@@ -66,7 +67,7 @@ type MissingValueStruct struct {
 }
 
 func MissingCreator(typ Type, addr Addr) Value {
-	return &MissingValueStruct{ Common{IntKind, typ, addr} }
+	return &MissingValueStruct{ Common{MissingKind, typ, addr} }
 }
 
 // -- Int