Warn when `value` and `defaultValue` are both specified

[jimfb]

This should be illegal:

ReactDOM.render(
  <input value="foo" defaultValue="foo1" onChange={function(){}} />,
  document.getElementById('container')
);

Users need to decide if they are using controlled or uncontrolled inputs. We should warn if both properties are specified.

[quantizor]

👍 I've run into this before and it can trip up new React developers.

[syranide]

@jimfb I might be reading #5680 wrong, but doesn't that make it valid to use both? (Although, HTML defaultValue really should be separate from React defaultValue right?)

[jimfb]

@syranide No, afaik, #5680 is trying to fix #4618 when using uncontrolled components. defaultValue is always meaningless for controlled components. #5680 does add a unit test that includes an element with both value and defaultValue, but I complained about that and hopefully it gets fixed before that gets merged (seeing that PR was the reason I created this issue).

[syranide]

@jimfb Ah ok I see now... Hmm, I don't advocate it at all, but if that PR is meant to fix <input type="reset" />, wouldn't it make sense to support both value + defaultValue, defaultValue being what it resets to when you hit reset... rather than the value it already has.

Anyway, wrong issue to have this discussion in. 👍