From 09d04edfcf2fef78e84dca5e841e99d86a8536af Mon Sep 17 00:00:00 2001
From: Robert Griesemer <gri@golang.org>
Date: Tue, 23 Jul 2013 14:18:38 -0700
Subject: [PATCH] go.tools/go/types: declare typeswitch local variable in
 correct scope

R=adonovan
CC=golang-dev
https://golang.org/cl/11534044
---
 go/types/stmt.go            | 23 +++++++++++------------
 go/types/testdata/stmt0.src | 14 ++++++++++++++
 2 files changed, 25 insertions(+), 12 deletions(-)

diff --git a/go/types/stmt.go b/go/types/stmt.go
index 19e88fbf..5930a3c3 100644
--- a/go/types/stmt.go
+++ b/go/types/stmt.go
@@ -381,12 +381,6 @@ func (check *checker) stmt(s ast.Stmt) {
 			return
 		}
 
-		var obj Object
-		if lhs != nil {
-			obj = NewVar(lhs.Pos(), check.pkg, lhs.Name, x.typ)
-			check.declare(check.topScope, lhs, obj)
-		}
-
 		check.multipleDefaults(s.Body.List)
 		for _, s := range s.Body.List {
 			clause, _ := s.(*ast.CaseClause)
@@ -411,14 +405,19 @@ func (check *checker) stmt(s ast.Stmt) {
 				}
 			}
 			check.openScope(clause)
-			// If lhs exists, declare a corresponding object in the case-local scope if necessary.
+			// If lhs exists, declare a corresponding variable in the case-local scope if necessary.
 			if lhs != nil {
-				// A single-type case clause implicitly declares a new variable shadowing lhs.
-				if len(clause.List) == 1 && typ != nil {
-					obj := NewVar(lhs.Pos(), check.pkg, lhs.Name, typ)
-					check.declare(check.topScope, nil, obj)
-					check.recordImplicit(clause, obj)
+				// spec: "The TypeSwitchGuard may include a short variable declaration.
+				// When that form is used, the variable is declared at the beginning of
+				// the implicit block in each clause. In clauses with a case listing
+				// exactly one type, the variable has that type; otherwise, the variable
+				// has the type of the expression in the TypeSwitchGuard."
+				if len(clause.List) != 1 || typ == nil {
+					typ = x.typ
 				}
+				obj := NewVar(lhs.Pos(), check.pkg, lhs.Name, typ)
+				check.declare(check.topScope, nil, obj)
+				check.recordImplicit(clause, obj)
 			}
 			check.stmtList(clause.Body)
 			check.closeScope()
diff --git a/go/types/testdata/stmt0.src b/go/types/testdata/stmt0.src
index 7e098177..f00e98c6 100644
--- a/go/types/testdata/stmt0.src
+++ b/go/types/testdata/stmt0.src
@@ -229,6 +229,20 @@ func typeswitch0() {
 	}
 }
 
+// Test correct scope setup.
+// (no redeclaration errors expected in the type switch)
+func typeswitch1() {
+	var t I
+	switch t := t; t := t.(type) {
+	case nil:
+		var _ I = t
+	case T:
+		var _ T = t
+	default:
+		var _ I = t
+	}
+}
+
 func rangeloops() {
 	var (
 		x int