Add test case for inverting the negation

Not sure why anyone would do this, but it's covered now.

README.md

@@ -440,7 +440,7 @@ Tag                    | Description
 `required`             | If present, flag/arg is required.
 `optional`             | If present, flag/arg is optional.
 `hidden`               | If present, command or flag is hidden.
-`negatable`            | If present on a `bool` field, supports prefixing a flag with `--no-` to set flag to `false`
+`negatable`            | If present on a `bool` field, supports prefixing a flag with `--no-` to invert the default value
 `format:"X"`           | Format for parsing input, if supported.
 `sep:"X"`              | Separator for sequences (defaults to ","). May be `none` to disable splitting.
 `mapsep:"X"`           | Separator for maps (defaults to ";"). May be `none` to disable splitting.

context.go

@@ -579,9 +579,6 @@ func (c *Context) getValue(value *Value) reflect.Value {
 		}
 		c.values[value] = v
 	}
-	if value.Flag != nil && value.Flag.Negated {
-		v.SetBool(false)
-	}
 	return v
 }
 
@@ -629,7 +626,12 @@ func (c *Context) Apply() (string, error) {
 			panic("unsupported path ?!")
 		}
 		if value != nil {
-			value.Apply(c.getValue(value))
+			v := c.getValue(value)
+			if value.Flag != nil && value.Flag.Negated {
+				v.SetBool(!v.Bool())
+			}
+
+			value.Apply(v)
 		}
 	}
 

kong_test.go

@@ -369,6 +369,19 @@ func TestNegatedBooleanFlag(t *testing.T) {
 	require.Equal(t, false, cli.Cmd.Flag)
 }
 
+func TestInvertedNegatedBooleanFlag(t *testing.T) {
+	var cli struct {
+		Cmd struct {
+			Flag bool `kong:"default='true',negatable"`
+		} `kong:"cmd"`
+	}
+
+	p := mustNew(t, &cli)
+	_, err := p.Parse([]string{"cmd", "--no-flag=false"})
+	require.NoError(t, err)
+	require.Equal(t, true, cli.Cmd.Flag)
+}
+
 func TestInvalidNegatedNonBool(t *testing.T) {
 	var cli struct {
 		Cmd struct {