LLD used to handle comdats as if the selection field was always set to IMAGE_COMDAT_SELECT_ANY. This means for obj files produced by cl /Gy, LLD would never report a duplicate symbol error.
- adds validation for the Selection field (should make no difference in practice for compiler-generated obj inputs)
- rejects comdats that have different Selection fields in different obj files (likewise). This is a bit more strict but also more self-consistent thank link.exe (see comment in code)
- implements handling for all the selection kinds
In practice, compilers only generate comdats with IMAGE_COMDAT_SELECT_NODUPLICATES (LLD now produces duplicate symbol errors for these), IMAGE_COMDAT_SELECT_ANY (no behavior change), and IMAGE_COMDAT_SELECT_LARGEST (for RTTI data, here LLD should no longer create broken executables when linking some TUs with RTTI enabled and some with it disabled – but see below).
The implementation of IMAGE_COMDAT_SELECT_LARGEST is incomplete: If one SELECT_LARGEST comdat replaces an earlier one, the comdat symbol is replaced correctly, but the old section stays loaded and if /opt:ref is disabled (via /opt:noref or /debug) it's still written to the output. That's not ideal, but better than the current treatment of just picking any one of those comdats. I hope to fix this better later.