fix: traverse.visitors.merge unable to handle enter,exit

[liuxingbaoyu]

Q	A
Fixed Issues?	Fixes #15587
Patch: Bug Fix?	√
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	√
Documentation PR Link
Any Dependency Changes?
License	MIT

I'm not sure if this will be a breaking change, although it's a bugfix, since the state parameter didn't support enter and exit at all before then.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/54580/

[JLHwung]

Can we add a return assertion here? Like asserts value is VisitorHandler.

[JLHwung]

We explicitly disallow catch-all enter and exit in Babel plugins:

babel/packages/babel-core/src/config/validation/plugins.ts

Lines 43 to 49 in 2db3f55

	if (obj.enter || obj.exit) {
	throw new Error(
	`${msg(
	loc,
	)} cannot contain catch-all "enter" or "exit" handlers. Please target individual nodes.`,
	);
	}

If key is _exploded, then value is a visitor processed by explode. In this case, I think we should move the catch-all check before the visitor shape check, so we don't have to handle _exploded and _verified in assertVisitorHandler.

[JLHwung]

If visitor has enter or exit, then it is a catch all visitor. In this case we don't have to go through its properties, instead we can call wrapWithStateOrWrapper on .enter and .exit and push it to the rootVisitor. In this approach we can set _exploded and _verified directly.

[JLHwung]

nit: should be visitors.merge, or add a new nested describe block.

[liuxingbaoyu]

Unfortunately, I don't know how to fix it.🤦‍♂️

[liuxingbaoyu]

Superseded by #15702