fix: Fix nil pointer panic in getLateBoundSymbol under --noCheck

[Andre Fajar N (andre-fajar-n)]

Fixes #64669

Analysis

With --declaration --emitDeclarationOnly --noCheck, declaration emit can force late binding for a computed expando assignment
like:

const k = "bar";
foo[k] = 1;
function foo() {}

In that path, getLateBoundSymbol calls getMembersOfSymbol / getExportsOfSymbol on the symbol's parent. For a top-level
script, the parent can be nil (the SourceFile has no symbol), which panics:
panic: runtime error: invalid memory address or nil pointer dereference
Without --noCheck, checking usually late-binds earlier, so the nil parent path is less likely to be hit.

Fix

Guard on a non-nil parent before forcing late binding in getLateBoundSymbol. If the parent is nil, fall through to the existing
lateSymbol = symbol fallback instead of dereferencing nil.
Added compiler test noCheckExpandoElementAccessAssignment covering this flags + repro combination.

Notes

AI-assisted change (Cursor); I reviewed and verified the patch locally against the repro and the new harness test.

[Copilot]

🟢 Approval recommended

The targeted nil guard resolves the reported panic and is covered by an appropriate compiler regression test.

0 open findings

What changed in this PR

Fixes a nil-pointer panic during declaration-only emit with --noCheck for top-level computed expando assignments.

Changes:

• Guards late binding when the symbol parent is nil.
• Adds a compiler regression test and output baselines.

File	Description
tsc/​internal/​checker/​checker.go	Prevents dereferencing a nil parent during late binding.
tsc/​testdata/​tests/​cases/​compiler/​noCheckExpandoElementAccessAssignment.ts	Adds the regression case.
tsc/​testdata/​baselines/​reference/​compiler/​noCheckExpandoElementAccessAssignment.js	Captures declaration emit.
tsc/​testdata/​baselines/​reference/​compiler/​noCheckExpandoElementAccessAssignment.symbols	Captures symbol output.
tsc/​testdata/​baselines/​reference/​compiler/​noCheckExpandoElementAccessAssignment.types	Captures inferred types.

🧠 Review effort: Balanced

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[Andre Fajar N (andre-fajar-n)]

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