Fix panic in the optional-chain transform when a chain ends in a tagged template

[bigboateng]

Fixes #63965

flattenChain in tsc/internal/transformers/estransforms/optionalchain.go walks inward until the chain head is a tagged template or carries a ?. token, but then unconditionally calls chain.Expression(). TaggedTemplateExpression has a Tag rather than an expression, so Node.Expression() panics with an unhandled-kind error.

// any target that lowers optional chaining, e.g. es2017
e?.``(

panic: Unhandled case in Node.Expression: KindTaggedTemplateExpression
  ...estransforms.flattenChain
      tsc/internal/transformers/estransforms/optionalchain.go:143

The input is malformed (TS1358 makes tagged templates in optional chains a grammar error), but the compiler should report the diagnostics rather than crash — the transform runs regardless because the error is recoverable.

• Use the tagged template's tag as the flattened chain base, and re-tag the template onto the nullish-checked value when rebuilding the chain (a tagged template can only be the head link, since flattenChain stops walking at it). The repro now emits e === null || e === void 0 ? void 0 : e``();.
• Add a regression test covering the minimized repro and sibling shapes (a?.bc``, a tagged template followed by a call, and property access on a tagged-template head).

This fix was developed with AI assistance; I reviewed, tested, and take responsibility for every line.

[bigboateng]

@microsoft-github-policy-service agree

[Copilot]

Pull request overview

Prevents optional-chain lowering from panicking when encountering malformed chains containing tagged templates.

Changes:

• Handles tagged templates when flattening and rebuilding optional chains.
• Adds regression cases and diagnostic, emit, type, and symbol baselines.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tsc/internal/transformers/estransforms/optionalchain.go	Supports tagged-template chain segments.
tsc/testdata/tests/cases/compiler/optionalChainTaggedTemplateNoCrash.ts	Adds regression scenarios.
tsc/testdata/baselines/reference/compiler/optionalChainTaggedTemplateNoCrash.errors.txt	Captures expected diagnostics.
tsc/testdata/baselines/reference/compiler/optionalChainTaggedTemplateNoCrash.js	Captures downlevel emit.
tsc/testdata/baselines/reference/compiler/optionalChainTaggedTemplateNoCrash.symbols	Captures symbol information.
tsc/testdata/baselines/reference/compiler/optionalChainTaggedTemplateNoCrash.types	Captures inferred types.

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[Daniel Rosenwasser (DanielRosenwasser)]

Nit: can we rename this to something that doesn't say "no crash"?

[bigboateng]

Renamed to optionalChainTaggedTemplate.ts.

[Daniel Rosenwasser (DanielRosenwasser)]

Maybe we should ensure that we do the right thing when we don't transform optional chaining away by adding a second target:

Suggested change

	// @target: es2017
	// @filename: repro.ts
	e?.``(
	// @filename: siblings.ts
	// @target: es2017, es2024
	// @filename: repro.ts
	e?.``(
	// @filename: siblings.ts

Not sure what others think.

[bigboateng]

Good idea, applied as suggested. The es2024 baselines show the chain reaching emit untransformed. The printer keeps ?. on the property access ((a?.b) c), but it drops `?.` on the tagged template itself (`e ();`). There is no legal syntax for a tagged template in an optional chain, which is why TS1358 exists. The output matches what 5.9 emits for the same input, so this is existing behavior, now pinned by the test.