go/types: expose Default function, which converts untyped T to T

Change-Id: Ibcf5e0ba694b280744a00c2c6fda300f0a653455
Reviewed-on: https://go-review.googlesource.com/30715Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/assignments.go

@@ -41,7 +41,7 @@ func (check *Checker) assignment(x *operand, T Type, context string) {
 				x.mode = invalid
 				return
 			}
-			target = defaultType(x.typ)
+			target = Default(x.typ)
 		}
 		check.convertUntyped(x, target)
 		if x.mode == invalid {
@@ -116,7 +116,7 @@ func (check *Checker) initVar(lhs *Var, x *operand, context string) Type {
 				lhs.typ = Typ[Invalid]
 				return nil
 			}
-			typ = defaultType(typ)
+			typ = Default(typ)
 		}
 		lhs.typ = typ
 	}

src/go/types/builtins.go

@@ -632,7 +632,7 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 func makeSig(res Type, args ...Type) *Signature {
 	list := make([]*Var, len(args))
 	for i, param := range args {
-		list[i] = NewVar(token.NoPos, nil, "", defaultType(param))
+		list[i] = NewVar(token.NoPos, nil, "", Default(param))
 	}
 	params := NewTuple(list...)
 	var result *Tuple

src/go/types/conversions.go

@@ -55,7 +55,7 @@ func (check *Checker) conversion(x *operand, T Type) {
 		//   not []byte as type for the constant "foo").
 		// - Keep untyped nil for untyped nil arguments.
 		if IsInterface(T) || constArg && !isConstType(T) {
-			final = defaultType(x.typ)
+			final = Default(x.typ)
 		}
 		check.updateExprType(x.expr, final, true)
 	}

src/go/types/expr.go

@@ -541,7 +541,7 @@ func (check *Checker) convertUntyped(x *operand, target Type) {
 			if !t.Empty() {
 				goto Error
 			}
-			target = defaultType(x.typ)
+			target = Default(x.typ)
 		}
 	case *Pointer, *Signature, *Slice, *Map, *Chan:
 		if !x.isNil() {
@@ -605,8 +605,8 @@ func (check *Checker) comparison(x, y *operand, op token.Token) {
 		// time will be materialized. Update the expression trees.
 		// If the current types are untyped, the materialized type
 		// is the respective default type.
-		check.updateExprType(x.expr, defaultType(x.typ), true)
-		check.updateExprType(y.expr, defaultType(y.typ), true)
+		check.updateExprType(x.expr, Default(x.typ), true)
+		check.updateExprType(y.expr, Default(y.typ), true)
 	}
 
 	// spec: "Comparison operators compare two operands and yield

src/go/types/predicates.go

@@ -291,11 +291,11 @@ func identical(x, y Type, cmpTags bool, p *ifacePair) bool {
 	return false
 }
 
-// defaultType returns the default "typed" type for an "untyped" type;
+// Default returns the default "typed" type for an "untyped" type;
 // it returns the incoming type for all other types. The default type
 // for untyped nil is untyped nil.
 //
-func defaultType(typ Type) Type {
+func Default(typ Type) Type {
 	if t, ok := typ.(*Basic); ok {
 		switch t.kind {
 		case UntypedBool: