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Andrew Clark
ed3c65caf0
We want to warn if we detect that an app is using an outdated JSX transform. We can't just warn if `createElement` is called because we still support `createElement` when it's called manually. We only want to warn if `createElement` is output by the compiler. The heuristic is to check for a `__self` prop, which is an optional, internal prop that older transforms used to pass to `createElement` for better debugging in development mode. If `__self` is present, we `console.warn` once with advice to upgrade to the modern JSX transform. Subsequent elements will not warn. There's a special case we have to account for: when a static "key" prop is defined _after_ a spread, the modern JSX transform outputs `createElement` instead of `jsx`. (This is because with `jsx`, a spread key always takes precedence over a static key, regardless of the order, whereas `createElement` respects the order.) To avoid a false positive warning, we skip the warning whenever a `key` prop is present.