## Summary We rely on `-undefined dynamic_lookup` to compile the `rts` with references that will be solved at dynamic linking time to some symbols e.g. from `ghc-internal`, given such as ``` ... "-Wl,-u,_hs_atomicwrite32" "-Wl,-u,_hs_atomicwrite64" "-Wl,-u,_ghczminternal_GHCziInternalziStackziCloneStack_StackSnapshot_closure" ``` On macOS 15.5 Sequoia, `-undefined dynamic_lookup` behaves as described (in the manpage), however, in macOS 26 Tahoe, `-undefined dynamic_lookup` is seemingly ignored and behaves like `-undefined error`. This was originally reported under #26152 by a user that failed to build GHC on the new macOS version. ## Steps to reproduce Write a main.c file ``` int main(void) { return 0; } ``` Compile with ``` gcc main.c -Wl,-undefined,dynamic_lookup -Wl,-u,mytest ``` Observe this works on macOS 15 but fails on macOS 26 beta. This also fails with `-Wl,-no_fixup_chains`. The error message looks something like: ``` Undefined symbols for architecture arm64: "mytest", referenced from: <initial-undefines> ld: symbol(s) not found for architecture arm64 ```