starlark: disallow None < None comparisons (google#314)

See spec change bazelbuild/starlark#116

starlark/eval_test.go

@@ -509,7 +509,7 @@ func TestBacktrace(t *testing.T) {
 	// functions, including propagation through built-ins such as 'min'.
 	const src = `
 def f(x): return 1//x
-def g(x): f(x)
+def g(x): return f(x)
 def h(): return min([1, 2, 0], key=g)
 def i(): return h()
 i()
@@ -522,7 +522,7 @@ i()
   crash.star:5:18: in i
   crash.star:4:20: in h
   <builtin>: in min
-  crash.star:3:12: in g
+  crash.star:3:19: in g
   crash.star:2:19: in f
 Error: floored division by zero`
 	if got := backtrace(t, err); got != want {

starlark/testdata/misc.star

@@ -39,6 +39,7 @@
 load("assert.star", "assert")
 
 # Ordered comparisons require values of the same type.
+assert.fails(lambda: None < None, "not impl")
 assert.fails(lambda: None < False, "not impl")
 assert.fails(lambda: False < list, "not impl")
 assert.fails(lambda: list < {}, "not impl")

starlark/value.go

@@ -132,7 +132,6 @@ type Comparable interface {
 }
 
 var (
-	_ Comparable = None
 	_ Comparable = Int{}
 	_ Comparable = False
 	_ Comparable = Float(0)
@@ -354,9 +353,6 @@ func (NoneType) Type() string          { return "NoneType" }
 func (NoneType) Freeze()               {} // immutable
 func (NoneType) Truth() Bool           { return False }
 func (NoneType) Hash() (uint32, error) { return 0, nil }
-func (NoneType) CompareSameType(op syntax.Token, y Value, depth int) (bool, error) {
-	return threeway(op, 0), nil
-}
 
 // Bool is the type of a Starlark bool.
 type Bool bool