Print parentheses around identifier let where necessary

[Zalathar]

Q	A
Fixed Issues?	Fixes #13268
Patch: Bug Fix?	Yes
Major: Breaking Change?	No
Minor: New Feature?	No
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?	No
License	MIT

If we detect that the identifier let would be printed in a location that forbids it (either unconditionally or when followed by [), print parentheses around the identifier so that it remains legal and correct.

The forbidden locations are:

• ExpressionStatement (let [)
• ForStatement (let [)
• ForInStatement (let [)
• ForOfStatement (let) (regular and await)

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/45877/

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 1aea906:

Sandbox	Source
babel-repl-custom-plugin	Configuration
babel-plugin-multi-config	Configuration

[Zalathar]

Ah, I just remembered that OptionalMemberExpression can also print a left bracket if { computed: true, optional: false }.

I don't think this can occur in head position in a freshly-parsed AST, but it can occur if a transform rearranges nodes and also sets node.optional = false.

Adding the appropriate check is easy enough, but I'll need to write a non-fixture test to get coverage.

[nicolo-ribaudo]

Ideally we should add some validation to ensure that optional: false can only happen if it has a (deep) child with optional: true, but adding the generator check is a good idea.

[nicolo-ribaudo]

Thanks!