In the standard library, we have many pieces like void foo(thread int*); void foo(device int*); void foo(threadgroup int*); if you say foo(nullptr), you get a compile error because it doesn't know which address space nullptr prefers.
Migrated to https://api.github.com/repos/gpuweb/WHLSL/issues/16
https://github.com/gpuweb/WHLSL/issues/16