starlark: tweak how UnpackArgs enforces optional args

Fixes google#544

Signed-off-by: Nick Santos <nick.santos@docker.com>

starlark/eval_test.go

@@ -1109,3 +1109,44 @@ f(1)
 		t.Errorf("env() returned %s, want %s", got, want)
 	}
 }
+
+func TestUnpackArgsOptionalInference(t *testing.T) {
+	// success
+	kwargs := []starlark.Tuple{
+		{starlark.String("x"), starlark.MakeInt(1)},
+		{starlark.String("y"), starlark.MakeInt(2)},
+	}
+	var x, y, z int
+	if err := starlark.UnpackArgs("unpack", nil, kwargs,
+		"x", &x, "y?", &y, "z", &z); err != nil {
+		t.Errorf("UnpackArgs failed: %v", err)
+	}
+	if x != 1 {
+		t.Errorf("for x, got %v, want %v", x, 1)
+	}
+	if y != 2 {
+		t.Errorf("for y, got %v, want %v", y, 2)
+	}
+	if z != 0 {
+		t.Errorf("for z, got %v, want %v", z, 0)
+	}
+
+	// success
+	args := starlark.Tuple{starlark.MakeInt(1), starlark.MakeInt(2)}
+	x = 0
+	y = 0
+	z = 0
+	if err := starlark.UnpackArgs("unpack", args, nil,
+		"x", &x, "y?", &y, "z", &z); err != nil {
+		t.Errorf("UnpackArgs failed: %v", err)
+	}
+	if x != 1 {
+		t.Errorf("for x, got %v, want %v", x, 1)
+	}
+	if y != 2 {
+		t.Errorf("for y, got %v, want %v", y, 2)
+	}
+	if z != 0 {
+		t.Errorf("for z, got %v, want %v", z, 0)
+	}
+}

starlark/unpack.go

@@ -44,23 +44,23 @@ type Unpacker interface {
 //
 // Examples:
 //
-//      var (
-//          a Value
-//          b = MakeInt(42)
-//          c Value = starlark.None
-//      )
+//	var (
+//	    a Value
+//	    b = MakeInt(42)
+//	    c Value = starlark.None
+//	)
 //
-//      // 1. mixed parameters, like def f(a, b=42, c=None).
-//      err := UnpackArgs("f", args, kwargs, "a", &a, "b?", &b, "c?", &c)
+//	// 1. mixed parameters, like def f(a, b=42, c=None).
+//	err := UnpackArgs("f", args, kwargs, "a", &a, "b?", &b, "c?", &c)
 //
-//      // 2. keyword parameters only, like def f(*, a, b, c=None).
-//      if len(args) > 0 {
-//              return fmt.Errorf("f: unexpected positional arguments")
-//      }
-//      err := UnpackArgs("f", args, kwargs, "a", &a, "b?", &b, "c?", &c)
+//	// 2. keyword parameters only, like def f(*, a, b, c=None).
+//	if len(args) > 0 {
+//	        return fmt.Errorf("f: unexpected positional arguments")
+//	}
+//	err := UnpackArgs("f", args, kwargs, "a", &a, "b?", &b, "c?", &c)
 //
-//      // 3. positional parameters only, like def f(a, b=42, c=None, /) in Python 3.8.
-//      err := UnpackPositionalArgs("f", args, kwargs, 1, &a, &b, &c)
+//	// 3. positional parameters only, like def f(a, b=42, c=None, /) in Python 3.8.
+//	err := UnpackPositionalArgs("f", args, kwargs, 1, &a, &b, &c)
 //
 // More complex forms such as def f(a, b=42, *args, c, d=123, **kwargs)
 // require additional logic, but their need in built-ins is exceedingly rare.
@@ -79,17 +79,16 @@ type Unpacker interface {
 // for the zero values of variables of type *List, *Dict, Callable, or
 // Iterable. For example:
 //
-//      // def myfunc(d=None, e=[], f={})
-//      var (
-//          d Value
-//          e *List
-//          f *Dict
-//      )
-//      err := UnpackArgs("myfunc", args, kwargs, "d?", &d, "e?", &e, "f?", &f)
-//      if d == nil { d = None; }
-//      if e == nil { e = new(List); }
-//      if f == nil { f = new(Dict); }
-//
+//	// def myfunc(d=None, e=[], f={})
+//	var (
+//	    d Value
+//	    e *List
+//	    f *Dict
+//	)
+//	err := UnpackArgs("myfunc", args, kwargs, "d?", &d, "e?", &e, "f?", &f)
+//	if d == nil { d = None; }
+//	if e == nil { e = new(List); }
+//	if f == nil { f = new(Dict); }
 func UnpackArgs(fnname string, args Tuple, kwargs []Tuple, pairs ...interface{}) error {
 	nparams := len(pairs) / 2
 	var defined intset
@@ -164,12 +163,15 @@ kwloop:
 	}
 
 	// Check that all non-optional parameters are defined.
-	// (We needn't check the first len(args).)
-	for i := len(args); i < nparams; i++ {
+	for i := 0; i < nparams; i++ {
 		name := pairs[2*i].(string)
 		if strings.HasSuffix(name, "?") {
 			break // optional
 		}
+		// (We needn't check the first len(args).)
+		if i < len(args) {
+			continue
+		}
 		if !defined.get(i) {
 			return fmt.Errorf("%s: missing argument for %s", fnname, name)
 		}