go/parser, go/types: report invalid else branch in if statements

- Only accept valid if statement syntax in go/parser.

- Check AST again in go/types since it may have been modified and the
  AST doesn't preclude other statements in the else branch of an if
  statement.

- Removed a test from gofmt which verified that old-style if statements
  permitting any statement in the else branch were correctly reformatted.
  It's been years since we switched to the current syntax; no need to
  support this anymore.

- Added a comment to go/printer.

Fixes #13475.

Change-Id: Id2c8fbcc68b719cd511027d0412a37266cceed6b
Reviewed-on: https://go-review.googlesource.com/17408
Reviewed-by: Russ Cox <rsc@golang.org>

1 files changed, 10 insertions, 1 deletions

diff --git a/src/go/parser/parser.go b/src/go/parser/parser.go
index 73edaa0ab3..f3a26032ee 100644
--- a/src/go/parser/parser.go
+++ b/src/go/parser/parser.go
@@ -1857,7 +1857,16 @@ func (p *parser) parseIfStmt() *ast.IfStmt {
 	var else_ ast.Stmt
 	if p.tok == token.ELSE {
 		p.next()
-		else_ = p.parseStmt()
+		switch p.tok {
+		case token.IF:
+			else_ = p.parseIfStmt()
+		case token.LBRACE:
+			else_ = p.parseBlockStmt()
+			p.expectSemi()
+		default:
+			p.errorExpected(p.pos, "if statement or block")
+			else_ = &ast.BadStmt{From: p.pos, To: p.pos}
+		}
 	} else {
 		p.expectSemi()
 	}