Cannot export `declare`d functions in flow

Bug Report

• I would like to work on a fix!

Current behavior A clear and concise description of the behavior.

• REPL

Input Code

declare function foo(): void;
export { foo };

Expected behavior

This should parse without an error.

Babel Configuration

repros just using preset flow

Environment

• Babel version(s): v7.11.1

Possible Solution

This should work fine as it’s no different to either of these working cases

declare export function bar(): void;

declare var bam: string;
export { bam };

[nicolo-ribaudo]

I’m trying to fix this, and Flow’s scope handling of declare functions seems completely random:

// error
function A(): void {}
declare function A(): void;

// ok
declare function A(): void;
function A(): void {}

// ok
declare function A(): void;
function A(): void {}
declare function A(): void;

[bradzacher]

I get your reasoning - but:

• If you’re banning declare function foo(): void; export { foo };, then you should also ban the variable decl case: declare var bam: string; export { bam };.
  • Otherwise there is a distinct asymmetry in babel’s logic.
• Why is declare export function foo(): void allowed? By your reasoning it should be banned.
• Most importantly: flow declaration files ofc do these practices, which is where it’s used, and why it shouldn’t be banned.

I don’t think babel should be in the business of validating semantics this deeply