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At the moment, when one enables IPO, CMake silently opts for "thin" objects, i.e. ones which have the minimum required information for performing IPO - which assumes the rest of the build toolchain has maximal support for IPO (e.g. re-invoking a compiler). For GCC, this is -fno-fat-lto-objects, and for clang it's -flto=thin.

It is not reasonable to enforce this. One of the following needs to happen:

1. The INTERPROCEDURAL_OPTIMIZATION target property should have additional options, e.g. NONE, THIN, FAT/FULL.
2. There should be an additional target property for IPO "fatness"
3. INTERPROCEDURAL_OPTIMIZATION will not tell the compiler anything abut "fatness" and leave it up to the user.

I like option (1.), though option (2.) would maximize compatibility with previous CMake versions. Although... one could have options OFF, ON and THIN - different option type, but subsumes the previous range of options.