Invalid output when using function() (anonymous functions) in prop

[dddlr]

Describe the bug

We don't currently support anonymous functions in components that are not arrow functions. When we try to do this, we get very bizarre output. We should throw an error in these situations (if we decide not to support them).

To Reproduce

Variant 1

const Button = styled.div({
  width: function({ width, height = 16 }) {
    return `${width + height}px`;
  },
});

Variant 2

const Button = styled.div({
  width({ width, height = 16 }) {
    return `${width + height}px`;
  },
});

Workarounds

Workaround is to use an arrow function instead.

Expected behavior

There should be a build-time error.

Version

Tested with master branch as of 8add86d (compiled/babel-plugin 0.20.0)