Fix nil panic when stripInternal elides type alias RHS

tsc/internal/transformers/declarations/transform.go

@@ -1791,12 +1791,17 @@ func (tx *DeclarationTransformer) transformTopLevelDeclaration(input *ast.Node)
 
 func (tx *DeclarationTransformer) transformTypeAliasDeclaration(input *ast.TypeAliasDeclaration) *ast.Node {
 	tx.needsDeclare = false
+	t := tx.Visitor().Visit(input.Type)
+	if t == nil {
+		// stripInternal can elide an @internal type RHS; fall back like index signatures
+		t = tx.Factory().NewKeywordTypeNode(ast.KindAnyKeyword)
+	}
 	return tx.Factory().UpdateTypeAliasDeclaration(
 		input,
 		tx.ensureModifiers(input.AsNode()),
 		input.Name(),
 		tx.Visitor().VisitNodes(input.TypeParameters),
-		tx.Visitor().Visit(input.Type),
+		t,
 	)
 }
 

tsc/testdata/baselines/reference/compiler/declarationEmitStripInternalTypeAliasRHS.js

@@ -0,0 +1,15 @@
+//// [tests/cases/compiler/declarationEmitStripInternalTypeAliasRHS.ts] ////
+
+//// [declarationEmitStripInternalTypeAliasRHS.ts]
+interface Foo {}
+export type T =
+    /** @internal */
+    Foo;
+
+
+//// [declarationEmitStripInternalTypeAliasRHS.js]
+export {};
+
+
+//// [declarationEmitStripInternalTypeAliasRHS.d.ts]
+export type T = any;

tsc/testdata/baselines/reference/compiler/declarationEmitStripInternalTypeAliasRHS.symbols

@@ -0,0 +1,13 @@
+//// [tests/cases/compiler/declarationEmitStripInternalTypeAliasRHS.ts] ////
+
+=== declarationEmitStripInternalTypeAliasRHS.ts ===
+interface Foo {}
+>Foo : Symbol(Foo, Decl(declarationEmitStripInternalTypeAliasRHS.ts, 0, 0))
+
+export type T =
+>T : Symbol(T, Decl(declarationEmitStripInternalTypeAliasRHS.ts, 0, 16))
+
+    /** @internal */
+    Foo;
+>Foo : Symbol(Foo, Decl(declarationEmitStripInternalTypeAliasRHS.ts, 0, 0))
+

tsc/testdata/baselines/reference/compiler/declarationEmitStripInternalTypeAliasRHS.types

@@ -0,0 +1,10 @@
+//// [tests/cases/compiler/declarationEmitStripInternalTypeAliasRHS.ts] ////
+
+=== declarationEmitStripInternalTypeAliasRHS.ts ===
+interface Foo {}
+export type T =
+>T : Foo
+
+    /** @internal */
+    Foo;
+

tsc/testdata/tests/cases/compiler/declarationEmitStripInternalTypeAliasRHS.ts

@@ -0,0 +1,7 @@
+// @declaration: true
+// @stripInternal: true
+
+interface Foo {}
+export type T =
+    /** @internal */
+    Foo;