From 670642d389a3c1a90fad6016f91197125969376d Mon Sep 17 00:00:00 2001
From: Robert Griesemer <gri@golang.org>
Date: Thu, 3 Dec 2015 17:28:46 -0800
Subject: [PATCH] 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>
---
 src/cmd/gofmt/testdata/old.golden |  9 ---------
 src/cmd/gofmt/testdata/old.input  |  8 --------
 src/go/parser/parser.go           | 11 ++++++++++-
 src/go/parser/short_test.go       |  4 ++++
 src/go/printer/nodes.go           |  3 +++
 src/go/types/stmt.go              |  9 ++++++++-
 6 files changed, 25 insertions(+), 19 deletions(-)
 delete mode 100644 src/cmd/gofmt/testdata/old.golden
 delete mode 100644 src/cmd/gofmt/testdata/old.input

diff --git a/src/cmd/gofmt/testdata/old.golden b/src/cmd/gofmt/testdata/old.golden
deleted file mode 100644
index 95a0b72a0e..0000000000
--- a/src/cmd/gofmt/testdata/old.golden
+++ /dev/null
@@ -1,9 +0,0 @@
-package P
-
-func f() {
-	if x {
-		y
-	} else {
-		z
-	}
-}
diff --git a/src/cmd/gofmt/testdata/old.input b/src/cmd/gofmt/testdata/old.input
deleted file mode 100644
index e24eed215d..0000000000
--- a/src/cmd/gofmt/testdata/old.input
+++ /dev/null
@@ -1,8 +0,0 @@
-package P
-
-func f() {
-	if x {
-		y
-	} else
-		z
-}
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()
 	}
diff --git a/src/go/parser/short_test.go b/src/go/parser/short_test.go
index e05ae8e9e9..cdd343ea3c 100644
--- a/src/go/parser/short_test.go
+++ b/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) {
diff --git a/src/go/printer/nodes.go b/src/go/printer/nodes.go
index 35c017db0e..11f26d45ea 100644
--- a/src/go/printer/nodes.go
+++ b/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)
diff --git a/src/go/types/stmt.go b/src/go/types/stmt.go
index 973af423c1..e0129cf0e0 100644
--- a/src/go/types/stmt.go
+++ b/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: