Fix nil panic when stripInternal elides type alias RHS

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

Fixes #64667

Analysis

With --declaration --stripInternal, @internal on a type-alias RHS can be stripped by visitDeclarationSubtree, leaving a nil
Type on the alias:

interface Foo {}
export type T =
    /** @internal */
    Foo;

Declaration emit then called ast.GetTypeNodePrecedence with nil from emitTypeAliasDeclaration → emitTypeNode, causing:
panic: runtime error: invalid memory address or nil pointer dereference
Index signatures already fall back to any when the visited type is nil; type aliases did not.

Fix

In transformTypeAliasDeclaration, if Visit(input.Type) returns nil, substitute any (same pattern as
transformIndexSignatureDeclaration). Added declarationEmitStripInternalTypeAliasRHS.ts and baselines.
Alternate considered: nil-guard only in emitTypeNode / GetTypeNodePrecedence. That would avoid the crash but could emit an
incomplete alias; transforming to any matches existing stripInternal declaration behavior.

AI Assistance

This PR was authored with AI assistance (Cursor); I have reviewed and understand the changes.

[Copilot]

🟢 Approval recommended

The focused fallback matches existing index-signature behavior and the regression baseline verifies export type T = any.

0 open findings

What changed in this PR

Fixes declaration emit panics when stripInternal removes a type alias RHS.

Changes:

• Falls back to any when the transformed RHS is nil.
• Adds a focused compiler regression test and baselines.

File	Description
tsc/​internal/​transformers/​declarations/​transform.go	Adds the any fallback.
tsc/​testdata/​tests/​cases/​compiler/​declarationEmitStripInternalTypeAliasRHS.ts	Reproduces the panic.
tsc/​testdata/​baselines/​reference/​compiler/​declarationEmitStripInternalTypeAliasRHS.js	Verifies declaration output.
tsc/​testdata/​baselines/​reference/​compiler/​declarationEmitStripInternalTypeAliasRHS.symbols	Records symbol baseline.
tsc/​testdata/​baselines/​reference/​compiler/​declarationEmitStripInternalTypeAliasRHS.types	Records type baseline.

🧠 Review effort: Balanced

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