Preserve comments when downleveling arrow expression bodies

[Copilot]

Downlevel optional chaining can hoist temporaries from an arrow expression body, forcing the body to become a block with a synthesized return. When the expression had a leading comment, that comment emitted after return, triggering ASI and skipping the expression.

• Emit fix

  • Propagate the original expression range to the synthesized return statement.
  • This keeps leading comments attached before return instead of between return and the returned expression.

• Regression coverage

  • Added an ES2018 compiler test covering an arrow expression body with a leading comment and optional chaining.

const wat = () =>
    // explanatory comment
    thing?.nested?.condition ? "pass" : "fail";

Now emits with the comment before return, avoiding return\n... ASI behavior.

• Fixes Nested nullish coalescing + comment + ES2018 causes function body to be ignored #4722

[Copilot]

Pull request overview

Preserves leading comments when arrow expression bodies are downleveled into blocks, preventing ASI from skipping returned expressions.

Changes:

• Copies the expression source range to synthesized return statements.
• Adds compiler regression coverage and baselines.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
internal/printer/emitcontext.go	Preserves the expression range on synthesized returns.
testdata/tests/cases/compiler/optionalChainingArrowFunctionCommentEs2018.ts	Adds the regression case.
testdata/baselines/reference/compiler/optionalChainingArrowFunctionCommentEs2018.js	Verifies correct comment placement in emitted JavaScript.
testdata/baselines/reference/compiler/optionalChainingArrowFunctionCommentEs2018.types	Records type output.
testdata/baselines/reference/compiler/optionalChainingArrowFunctionCommentEs2018.symbols	Records symbol output.

[weswigham]

This clones the expression's pos to the return statement - which is fine IF you also suppress comments on updated. As it is now, there may be duplicated comments in some situations (inline comments, likely).

[jakebailey]

@copilot think harder, ground yourself in Strada's code, etc etc

[Copilot]

Addressed in 38e7d06. I now suppress comments on the rewritten expression when synthesizing the return in VisitFunctionBody, so we keep the ASI-safe return positioning without re-emitting comments from updated.