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/17408Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/gofmt/testdata/old.golden

-package P
-
-func f() {
-	if x {
-		y
-	} else {
-		z
-	}
-}

src/cmd/gofmt/testdata/old.input

-package P
-
-func f() {
-	if x {
-		y
-	} else
-		z
-}

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()
 	}

src/go/parser/short_test.go

@@ -121,6 +121,10 @@ var invalids = []string{
 	`package p; type _ struct { ( /* ERROR "expected anonymous field" */ int) };`,
 	`package p; func _()(x, y, z ... /* ERROR "expected '\)', found '...'" */ int){}`,
 	`package p; func _()(... /* ERROR "expected type, found '...'" */ int){}`,
+
+	// issue 13475
+	`package p; func f() { if true {} else ; /* ERROR "expected if statement or block" */ }`,
+	`package p; func f() { if true {} else defer /* ERROR "expected if statement or block" */ f() }`,
 }
 
 func TestInvalid(t *testing.T) {

src/go/printer/nodes.go

@@ -1185,6 +1185,9 @@ func (p *printer) stmt(stmt ast.Stmt, nextIsRBrace bool) {
 			case *ast.BlockStmt, *ast.IfStmt:
 				p.stmt(s.Else, nextIsRBrace)
 			default:
+				// This can only happen with an incorrectly
+				// constructed AST. Permit it but print so
+				// that it can be parsed without errors.
 				p.print(token.LBRACE, indent, formfeed)
 				p.stmt(s.Else, true)
 				p.print(unindent, formfeed, token.RBRACE)

src/go/types/stmt.go

@@ -457,8 +457,15 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 			check.error(s.Cond.Pos(), "non-boolean condition in if statement")
 		}
 		check.stmt(inner, s.Body)
-		if s.Else != nil {
+		// The parser produces a correct AST but if it was modified
+		// elsewhere the else branch may be invalid. Check again.
+		switch s.Else.(type) {
+		case nil, *ast.BadStmt:
+			// valid or error already reported
+		case *ast.IfStmt, *ast.BlockStmt:
 			check.stmt(inner, s.Else)
+		default:
+			check.error(s.Else.Pos(), "invalid else branch in if statement")
 		}
 
 	case *ast.SwitchStmt: