reflect.DeepEqual: don't panic comparing functions

Functions are equal iff they are both nil.

Fixes #3122.

R=golang-dev, dsymonds, rsc
CC=golang-dev
https://golang.org/cl/5693057

src/pkg/reflect/all_test.go

@@ -629,6 +629,13 @@ type DeepEqualTest struct {
 	eq   bool
 }
 
+// Simple functions for DeepEqual tests.
+var (
+	fn1 func()             // nil.
+	fn2 func()             // nil.
+	fn3 = func() { fn1() } // Not nil.
+)
+
 var deepEqualTests = []DeepEqualTest{
 	// Equalities
 	{1, 1, true},
@@ -641,6 +648,7 @@ var deepEqualTests = []DeepEqualTest{
 	{Basic{1, 0.5}, Basic{1, 0.5}, true},
 	{error(nil), error(nil), true},
 	{map[int]string{1: "one", 2: "two"}, map[int]string{2: "two", 1: "one"}, true},
+	{fn1, fn2, true},
 
 	// Inequalities
 	{1, 2, false},
@@ -658,6 +666,8 @@ var deepEqualTests = []DeepEqualTest{
 	{map[int]string{2: "two", 1: "one"}, map[int]string{1: "one"}, false},
 	{nil, 1, false},
 	{1, nil, false},
+	{fn1, fn3, false},
+	{fn3, fn3, false},
 
 	// Nil vs empty: not the same.
 	{[]int{}, []int(nil), false},

src/pkg/reflect/deepequal.go

@@ -108,6 +108,12 @@ func deepValueEqual(v1, v2 Value, visited map[uintptr]*visit, depth int) (b bool
 			}
 		}
 		return true
+	case Func:
+		if v1.IsNil() && v2.IsNil() {
+			return true
+		}
+		// Can't do better than this:
+		return false
 	default:
 		// Normal equality suffices
 		return valueInterface(v1, false) == valueInterface(v2, false)
@@ -117,8 +123,8 @@ func deepValueEqual(v1, v2 Value, visited map[uintptr]*visit, depth int) (b bool
 }
 
 // DeepEqual tests for deep equality. It uses normal == equality where possible
-// but will scan members of arrays, slices, and fields of structs. It correctly
-// handles recursive types.
+// but will scan members of arrays, slices, maps, and fields of structs. It correctly
+// handles recursive types. Functions are equal only if they are both nil.
 func DeepEqual(a1, a2 interface{}) bool {
 	if a1 == nil || a2 == nil {
 		return a1 == a2