fix(printer): clamp out-of-range pos in getLineAndCharacter for source map emit

[Zelys-DFKH]

Fixes #3900

Analysis

tsgo --sourceMap on any file that triggers a parse error producing a synthetic closing token will panic:

echo '{' > bug.ts && tsgo --sourceMap bug.ts
# panic: runtime error: slice bounds out of range [:3] with length 2

The crash is in lineCharacterCache.getLineAndCharacter (internal/printer/utilities.go). It computes source map character offsets by slicing the source text: c.text[cachedPos:pos]. When the parser encounters an unclosed {, it inserts a synthetic } at a position past the end of the file (position 3 for a 2-byte file, {\n). Go panics on the out-of-bounds slice.

TypeScript's original implementation doesn't have this problem because String.prototype.slice silently clamps its end index to string.length. The Go port replicates the algorithm but not that implicit clamp.

Fix

One guard at the top of getLineAndCharacter: if pos > len(c.text), clamp it to len(c.text). Positions within bounds pass through unchanged, so this only affects synthetic tokens at or past end-of-file. The clamped position maps them to EOF in the source map, matching what TypeScript's JS implementation produces.

Copilot Checklist

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[Copilot]

Pull request overview

Fixes a panic in source map emission when the parser produces synthetic tokens positioned past end-of-file (e.g. an auto-inserted } for an unclosed block). The Go port's getLineAndCharacter sliced c.text[cachedPos:pos] directly, which panics on out-of-bounds, unlike JS's String.prototype.slice which clamps. The fix clamps pos to len(c.text).

Changes:

• Clamp pos to len(c.text) at the top of lineCharacterCache.getLineAndCharacter.
• Add a new compiler test sourceMapEmitUnclosedBlock.ts plus accepted baselines.

Reviewed changes

Copilot reviewed 6 out of 8 changed files in this pull request and generated no comments.

File	Description
internal/printer/utilities.go	Adds bounds clamp for out-of-range positions in getLineAndCharacter.
testdata/tests/cases/compiler/sourceMapEmitUnclosedBlock.ts	New minimal repro test enabling --sourceMap on an unclosed block.
testdata/baselines/reference/compiler/sourceMapEmitUnclosedBlock.*	Accepted baselines (js, js.map, sourcemap.txt, errors.txt, types, symbols) for the new test.

Files not reviewed (1)

• testdata/baselines/reference/compiler/sourceMapEmitUnclosedBlock.js: Language not supported

[jakebailey]

We already have #3923.

[Zelys-DFKH]

Closing — #3923 already fixes this. Thanks for the quick catch, @jakebailey.