he/him 
In TC39 we discussed reserving : after a declarator so implementations could not de-facto standardize incompatible type annotations. We knew this would oblige us to standardize type anno/expr syntax at some point. We didn't figure on Flow using ambiguous expression syntax! @awbjs
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the first one would be a syntax error. The curly brackets can’t be empty the rest would treated as the string child of JSX.
I'm not sure what you're saying. Both lines are valid syntax in TypeScript and Flow. The first line is a JSX element with the string contents "() => {}". The second is an empty arrow function with an X type parameter. https://astexplorer.net/#/gist/b7ec982c0246be182a05ed0c7dbc3dd0/873d02db09fbe182d655663cbeff8d3c3b161244 …
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In the first case the {} part will error because it’ll think you’re trying to use a JavaScript expression. It’ll error because it’s empty.
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