This line can probably go. There's no harm to 0 vs 0px and some older browsers want the unit anyway.

@probablyup unitless zero is part of CSS 2.1 which is supported even in IE 8.
As i know, React supports IE 9+, which makes adding px to 0 senseless.
This file is mostly copied "as it" from react source code. I think it will be strange when react itself doesn't apply px to zero, but styled-components will do it.

Yes but it's not required so let's save the few bytes

I don't want to be nitpicky, but a few bytes that we save here is putting more strain on gzip; either way were talking 6-7 chars after uglification. The bulk is still the map of properties 😅

let's remove the variable assignment since it isn't being used again

I can't think of when this would ever happen, let's remove.

@probablyup it necessary for this:

    const Bar = styled.div`
      ${Foo}: {
        background-color: red;
      };
    `;

How so? Nothing in that block is a symbol.

@probablyup because flatten only warns about passing non styled component but doesn't break function execution. So, here https://github.com/styled-components/styled-components/blob/master/src/utils/flatten.js#L88 React component passed to objToCss where it loops over all React component properties.

Ah, let's do an isValidElement check to just skip processing that stuff then.

@probablyup maybe it will be better to have return in https://github.com/styled-components/styled-components/blob/master/src/utils/flatten.js#L73 ?

Or even throw an actual error...

@probablyup

How about to throw StyledError here in both dev/prod envs?

      if (process.env.NODE_ENV !== 'production') {
        /* Warn if not referring styled component */
        try {
          // eslint-disable-next-line new-cap
          if (isElement(new chunk(executionContext))) {
            console.warn(
              `${getComponentName(
                chunk
              )} is not a styled component and cannot be referred to via component selector. See https://www.styled-components.com/docs/advanced#referring-to-other-components for more details.`
            );
          }
          // eslint-disable-next-line no-empty
        } catch (e) {}
      }

Yeah, let's do it.

I think this can just be simplified to !(name in unitless)

This naming is a little ominous, let's change it to addUnitIfNeeded