go/ast: add {L,R}paren token.Pos field to ast.TypeAssertExpr.

These are required to correctly determine the End() of the node.

Also set these fields in go/parser and use them in go/printer.

This is a backward-compatible API change.

R=gri, r
CC=golang-dev
https://golang.org/cl/10189043

src/pkg/go/ast/ast.go

@@ -305,7 +305,9 @@ type (
 	//
 	TypeAssertExpr struct {
 		X      Expr      // expression
+		Lparen token.Pos // position of "("
 		Type   Expr      // asserted type; nil means type switch X.(type)
+		Rparen token.Pos // position of ")"
 	}
 
 	// A CallExpr node represents an expression followed by an argument list.
@@ -463,12 +465,7 @@ func (x *ParenExpr) End() token.Pos    { return x.Rparen + 1 }
 func (x *SelectorExpr) End() token.Pos   { return x.Sel.End() }
 func (x *IndexExpr) End() token.Pos      { return x.Rbrack + 1 }
 func (x *SliceExpr) End() token.Pos      { return x.Rbrack + 1 }
-func (x *TypeAssertExpr) End() token.Pos {
-	if x.Type != nil {
-		return x.Type.End()
-	}
-	return x.X.End()
-}
+func (x *TypeAssertExpr) End() token.Pos { return x.Rparen + 1 }
 func (x *CallExpr) End() token.Pos       { return x.Rparen + 1 }
 func (x *StarExpr) End() token.Pos       { return x.X.End() }
 func (x *UnaryExpr) End() token.Pos      { return x.X.End() }

src/pkg/go/parser/parser.go

@@ -1150,7 +1150,7 @@ func (p *parser) parseTypeAssertion(x ast.Expr) ast.Expr {
 		defer un(trace(p, "TypeAssertion"))
 	}
 
-	p.expect(token.LPAREN)
+	lparen := p.expect(token.LPAREN)
 	var typ ast.Expr
 	if p.tok == token.TYPE {
 		// type switch: typ == nil
@@ -1158,9 +1158,9 @@ func (p *parser) parseTypeAssertion(x ast.Expr) ast.Expr {
 	} else {
 		typ = p.parseType()
 	}
-	p.expect(token.RPAREN)
+	rparen := p.expect(token.RPAREN)
 
-	return &ast.TypeAssertExpr{X: x, Type: typ}
+	return &ast.TypeAssertExpr{X: x, Type: typ, Lparen: lparen, Rparen: rparen}
 }
 
 func (p *parser) parseIndexOrSlice(x ast.Expr) ast.Expr {

src/pkg/go/printer/nodes.go

@@ -754,13 +754,13 @@ func (p *printer) expr1(expr ast.Expr, prec1, depth int) {
 
 	case *ast.TypeAssertExpr:
 		p.expr1(x.X, token.HighestPrec, depth)
-		p.print(token.PERIOD, token.LPAREN)
+		p.print(token.PERIOD, x.Lparen, token.LPAREN)
 		if x.Type != nil {
 			p.expr(x.Type)
 		} else {
 			p.print(token.TYPE)
 		}
-		p.print(token.RPAREN)
+		p.print(x.Rparen, token.RPAREN)
 
 	case *ast.IndexExpr:
 		// TODO(gri): should treat[] like parentheses and undo one level of depth