This detects cases where module A imports a function from B, and C
imports that same function from A instead of B. It's not part of the API
contract of A, and causes innocent refactoring to break things.
D lang compilers have an option -release (or similar) which turns off
asserts, contracts, and other runtime type checking. This patch wires
that up to the b_ndebug flag.
Fixes#7082