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I'm not sure how we handle flow types, perhaps keep it optional?

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I'm not sure how we handle flow types, perhaps keep it optional?

I suggest we keep it as-is to align with the current flow parser AST: https://flow.org/try/#1N4Igxg9gdgZglgcxALlAIwIZoKYBsD6uEEAztvhgE6UYCe+JADpdhgCYowa5kA0I2KAFcAtiRQAXSkOz9sADwxgJ+NPTbYuQ3BMnTZA+Y2yU4IwRO4A6SFBIrGVDGM7c+h46fNRLuKxJIGWh8MeT0ZfhYlCStpHzNsFBAMIQkIEQwJODAQfiEyfCEoAGsoCAB3KHw4KrBMsAALfAA3KjgsXGxxZC4eAw1m-GhcWn9aY3wWZlc+-g1mbDqJbDYAWglxxJ63AxJi7AlG2lXhXFwZ9z2Do5PRHEoLgyKwTqp1zce8qChFrpIqY41VqmDA+T4gIo-MB-AGrCCMLLQbirRoYGrgor-GDYVYIA7LSgkVZkCQE7q9dxFfIrVbMdJwMjghSeBIhPwYEhkSgqeEmTIQB7bPoAX34kA0SQAFABKAAEAF4AHyy4DC3IgZomEhwaBJZoABisAGZ9QBWKwARhAwqAA

I'm not sure how we handle flow types, perhaps keep it optional?

I suggest we keep it as-is to align with the current flow parser AST: flow.org/try#1N4Igxg9gdgZglgcxALlAIwIZoKYBsD6uEEAztvhgE6UYCe+JADpdhgCYowa5kA0I2KAFcAtiRQAXSkOz9sADwxgJ+NPTbYuQ3BMnTZA+Y2yU4IwRO4A6SFBIrGVDGM7c+h46fNRLuKxJIGWh8MeT0ZfhYlCStpHzNsFBAMIQkIEQwJODAQfiEyfCEoAGsoCAB3KHw4KrBMsAALfAA3KjgsXGxxZC4eAw1m-GhcWn9aY3wWZlc+-g1mbDqJbDYAWglxxJ63AxJi7AlG2lXhXFwZ9z2Do5PRHEoLgyKwTqp1zce8qChFrpIqY41VqmDA+T4gIo-MB-AGrCCMLLQbirRoYGrgor-GDYVYIA7LSgkVZkCQE7q9dxFfIrVbMdJwMjghSeBIhPwYEhkSgqeEmTIQB7bPoAX34kA0SQAFABKAAEAF4AHyy4DC3IgZomEhwaBJZoABisAGZ9QBWKwARhAwqAA

The flow parser seems to always set this property.

The flow parser seems to always set this property.

Right, didn't you just mention that the expression property will be set when the flow option is enabled? So the AST does align with the flow.

Oh, I understand what you mean.

Will we preserve the flow property in the @babel/types definition? ArrowFunctionExpression.predicate is also a property specific to flow.
If we're going to keep them, perhaps we could make expression optional in @babel/types instead of removing it.

Will we preserve the flow property in the @babel/types definition?

Yes, we don't plan to drop flow support on Babel 8.

make expression optional in @babel/types instead of removing it.

That sounds good to me. We can just remove the manual sync part as long as current logic does not depend on the expression property.

Marking this as a bugfix because we already do not generate .expression by default. This is just updating @babel/types to reflect that.