Handle undefined symbol.declarations in cloneSymbol (#18474)

src/compiler/checker.ts

@@ -578,7 +578,7 @@ namespace ts {
 
         function cloneSymbol(symbol: Symbol): Symbol {
             const result = createSymbol(symbol.flags, symbol.escapedName);
-            result.declarations = symbol.declarations.slice(0);
+            result.declarations = symbol.declarations ? symbol.declarations.slice() : [];
             result.parent = symbol.parent;
             if (symbol.valueDeclaration) result.valueDeclaration = symbol.valueDeclaration;
             if (symbol.constEnumOnlyModule) result.constEnumOnlyModule = true;

tests/baselines/reference/mergedClassWithNamespacePrototype.errors.txt

@@ -0,0 +1,15 @@
+/b.ts(2,15): error TS2300: Duplicate identifier 'prototype'.
+
+
+==== /a.d.ts (0 errors) ====
+    declare class Foo {}
+    
+==== /b.ts (1 errors) ====
+    declare namespace Foo {
+        namespace prototype {
+                  ~~~~~~~~~
+!!! error TS2300: Duplicate identifier 'prototype'.
+            function f(): void;
+        }
+    }
+    

tests/baselines/reference/mergedClassWithNamespacePrototype.js

@@ -0,0 +1,14 @@
+//// [tests/cases/compiler/mergedClassWithNamespacePrototype.ts] ////
+
+//// [a.d.ts]
+declare class Foo {}
+
+//// [b.ts]
+declare namespace Foo {
+    namespace prototype {
+        function f(): void;
+    }
+}
+
+
+//// [b.js]

tests/cases/compiler/mergedClassWithNamespacePrototype.ts

@@ -0,0 +1,9 @@
+// @Filename: /a.d.ts
+declare class Foo {}
+
+// @Filename: /b.ts
+declare namespace Foo {
+    namespace prototype {
+        function f(): void;
+    }
+}