tests/cases/conformance/ambient/ambientExternalModuleInsideNonAmbientExternalModule.ts(1,23): error TS2435: Ambient external modules cannot be nested in other modules.
==== tests/cases/conformance/ambient/ambientExternalModuleInsideNonAmbientExternalModule.ts (1 errors) ====
export declare module "M" { }
~~~
!!! error TS2435: Ambient external modules cannot be nested in other modules.